I’m honestly always a bit unsure about this topic. From one side I don’t want to generalise and being ableist because:

If your critique is that he's dumb because he has a stutter

This is objectively wrong and I totally get it. On the other end, we don’t live in a perfect world and to convey some emotion we need to use words that have a baggage or are imperfect. In any case, my personal meter for judging is always the intention and the context, more than the word itself.

Do I think every person who shutters is dumb that’s why I used it on Biden? Of course not.

Do I find hilarious thinking at Biden in a pool of shit having an heart attack and trying to spell “help”? Absolutely yes.

I would have liked the meme more if on the Trump part there was “shits himself”.

It’s the contrast the point, putting a man of power who should project this idea of “alfa” man with the reality of normal human disability. It puts them at our level, where we can bully them.