I have to admit that sometimes I see novelists on the forums whose grammar makes their posts so incomprehensible that I feel compelled to see if their novel reads that badly (In most cases it does, unfortunately).
I think generally people are pretty clear that this is a space for casual discussion, not like... a literary journal. Nobody expects your forum posts to be free of typos or wonky sentence structures or whatever. That said, clear English does get you a long way in discussions, so there's nothing wrong with scrutinising your own writing to spot or fix mistakes.
Ultimately though, there's a big difference between writing in casual internet style deliberately like:
"idk man i thought their comic was p cool tho"
...and writing with just bad grammar because your English skills need work.
"I dont kno what U mean with all do respect i wouldnt of criticed there comic"
Even though they're both "improper" English, the second one is waaaay less comprehensible due to lack of punctuation in places it's really needed, and then some incorrect homonyms for good measure. 
