People with disabilities and chronic illnesses are often accused of faking their condition. It's a very common experience for a person with a chronic illness to have a good day where their symptoms are manageable, get some errands or something done, and then have people around them use it as a weapon against them "Oh you were able to walk just fine yesterday, what's your excuse now?". The idea that someone's level of ability might fluctuate over time, that symptoms might be better one day and worse the next, just doesn't occur to a lot of able bodied people. And the Calvinist/Chud around "all poor people are welfare queens and poverty and illness are a sign god hates you" really reinforces and supports that view within the culture. It's real bad for a lot of disabled people.

This mockery of Joe reflects that - Joe was able to get out of bed for one day to help his grandson, so obviously he must be a lazy shirker who is just pretending to disability. It's 1:1 with the accusations disabled people face constantly.