Isn't it racist, sexist, etc to assume that certain groups can't speak/write properly?
It's racist, sexist, etc. to assume the speech patterns specific to a group are Correct and the standard and everything else is to be measured against. E.g. things like AAVE or vocal fry are often considered "wrong" (as opposed to just a variation), and there's at least an argument that it's because they're associated with black people and women, respectively.
As for ableism, well, no. It's not at all ableist to, e.g. think someone who is dyslexic might have trouble with spelling.
Classism is something of a middle ground. speech patterns associated with being lower-class are devalued in the same way AAVE is, but there's also the fact that limited access to education will also result in having a more limited vocabulary, etc.
TL;DR: judging people for spelling and punctuation is pretty fucking silly. What you consider proper grammar and sentence structure can be influenced by prejudice, and even if in cases when it isn't use of language is a skill like any other. Some people have it and others don't. It's better to have it, but it doesn't determine your moral worth.
(and I didn't even get into how being bad at English is used to paint non-native speakers as idiots, because things start getting personal and nobody wants that)
