It's worth noting that in everyday speech we frequently make all kinds of grammatical errors: We leave sentences unfinished, we say "umm", we inappropriately modify existing words to suit our purpose instead of looking for a better one, we finish sentences with nonsensical statements such as "You know". It's worthwhile trying to write dialogue which reflects the fluidity of normal speech.
Spelling errors/typos need to be fixed though (and yes, I'm guilty of making a few of my own) - they serve as an obstacle to enjoying the comic because the reader is focused on *how* things are said, rather than *what* is said.
That only works if you want people to enjoy your webcomic.