I think people explained it horribly. This is my attempt. If you got it, ignore this and think of it as an explanation for everyone else.

Dude said he was against processed food, especially fast food, and he made it part of his political identity. Just like he made his stance against vaccines part of his political identity. There is a chain of events that highly suggest that he is partly responsible for the dead of (I think) 83 children, due to his stance against vaccines. He joined trump because trump promise him control over the department of health. Obviously, that is kinda his wet dream As he promised his supporters that he would go against vaccines, processed food, gmo,...

Now he eats McDonald's. He threw away part of his political identity. Part of why people supported him, because he can't say no to trump. He sold out.

That might be good for America, as trump isn't against vaccines. As trump expressed he would like to brag about the "fact" that over his presidency, they "found" a vaccine against COVID. But trump knows that COVID vaccine bad in his supporters mind. That doesn't mean over vaccines are. So maybe it is good that jfk Jr is betraying his political identity.

But of course, betraying your political identity right after an election, is a dick move.

I hope this helps