Is it not okay to call a politician an idiot or an asshole for doing something you don't like?

It's pretty fair to call a powerful individual an asshole if they're using the unequal power dynamic to fuck up peoples' lives. It's not even a comparison because you're presenting a disability as an excuse to ignore "wrong" behaviour, rather than reconsidering what behaviour is "wrong" in the first place. And when you identify a "wrong" behaviour, consider why bullying would be the "moral" thing to do.

Sure, someone with severe NPD being narcissistic and always feeling like the victim is about as given as someone with a severe motor function deficit not being able to function at certain tasks, and you can get really philosophical about how humans are deterministic, fault doesn't exist, and free will is an illusion, but... most people are obviously gonna feel a lot more lack of sympathy towards the person with NPD just on the basis of how intentional people feel it to be. People feel a lot more upset when they feel targetted by something. People are homophobic and bully people expressing "abnormal" characteristics because, to them, it's an attack on their culture norms. People bully those with NPD because they see it as an attack on basic morality or as a danger to others. People bully boys who express certain "weak" emotions because it's an attack on the traditional patriarchal idea of masculinity.

It is society's job to make sure that people who do "wrong" get help to better themselves and society. Whether to think if insults or bullying will have do good in the context is hard to tell for yourself.

Personally, I will often insult someone on the alt-right. Not because I think they intentionally turned out that way, and not because I think they as a human being deserve suffering, but because I want to send a message that their beliefs are wrong and unwelcome. I want them to associate their beliefs with "fringe" and "unscientific" so they don't feel comfortable spreading hate in public. Possibly it pushes those people to radicalize more, but it's hard to argue that it isn't effective at making most of them reluctant to spread conservative views in public and it makes quite a few of them eventually start to question their own views.

What if they have a condition that makes them behave in a way that you call idiotic or assholeish?

Insults are inherently irrational, using derogatory terms is never rational. There is no clear and objective way to determine if an insult is "justified", because justification and logical thinking is subjective. When using insults, it's important to consider the goal and results of the insult. Why are you calling a person an idiot? Is it because they're acting in a way which you find "weird" or "annoying" or even "aggressive", and you want to feel better about yourself or harm them emotionally?

In your example, the perspective is that a person holds power over others and is using it to cause harm in one way or another, and it's felt that the person doing it is in control of their actions, so even knowing that an insult will bring no improvements they just feel an emotional pressure to vent.

That's a scenario where it becomes obvious that justification is subjective and, even if it were reasonable to decide it's unintentional, some people just feel helpless or angry and want to take out their frustration on the person they feel is doing the actions.

You use bullying when you want to achieve something. Self-satisfaction, emotional manipulation, emotional abuse, fitting in with others, whatever. Bullying is a tool to try to shift power, to bring someone else down, whether it's to "level the playing field" more or to put someone below you.

So what goal do you have when you call someone who's stimming or fidgeting or breaking down crying a derogatory name? What goal do you have when you call a child an asshole for refusing to eat a certain food? What goal do you have when you have when you call a politician an idiot?

Are you trying to emotionally abuse them into acting a certain way? Are you trying to gain self-satisfaction/relief and express your frustration at their cost? Are you trying to change other peoples' perceptions of the person? Are you just trying to hurt / punish them with no end goal because you feel they deserve it since you got stressed by their behaviour?

It's ignorant to for intent to be the default assumption. Always assume that someone could be different from you and that they may not be at fault for something you don't like. Then consider things you wouldn't do if they happened to have some sort of difference from you. That's the basic idea of treating others with sensitivity. If you think a person has certain harmful beliefs or does certain harmful actions, would you treat them differently if they had ASD or dysthemia or NPD?

Treat anyone the same way you would treat them if they were neurodiverse. Your opinion of them shouldn't generally change if you were to someday know that they had ASD or something. That's an important way to stave off unjustified biases and treatments towards disadvantaged peoples.

A child refusing to even try to eat something their parent worked so hard to make could be considered assholeish behavior,

I mean if you're down with blaming the children for being whatever you think of as misbehaved... I think "kids are assholes" is a funny phrase but unjokingly pinning the fault on the kid for their behaviour as if they choose to be reluctant to eat and calling them an asshole is a whole nother level of unempathetic. Kids are not rational and shouldn't be bullied.

are we to assume it's because they have autism and thus never call them out on it?

Generally if children (or pets) refuse to eat it's because there's something causing them to, maybe it's stress or emotional trauma, or maybe it's sensitivities caused by a disorder, or maybe it's an underlying illness like ulcers or cancer, or maybe it's because they have a certain biological reaction to the food, or maybe literally anything else. The mere idea of "calling out" a child in the first place is dumb, as if they're to blame for what foods do or do not repulse them or what causes them stress.

What do you is be a good parent and support your child, helping them get through the obstacles they were given. Not get upset at them, subconsciously blame them, and then be surprised when that sort of attitude towards them emotionally pushes them away or traumatizes them. Which is a typical experience for people with Autism and ADHD because most people, and most parents, default to assigning fault to the person they associate with issues, and raise their child thinking of them as a burden.

This isn't all to say I'm above all these heathens and I don't make fun of people. But usually I try to treat everyone as if they're potentially neurodivergent unless I have a reason not to. So when I do insult someone, whether or not they have Autism or Bipolar would not matter much to my insult.

Ableism is caused by not having a disability is treated as the default, like how white racism is white being treated as the default, and how homophobia is straightness being treated as the default, and ...