I'm kinda a recovering grammar freak, and I consider it to be like any other kind of critique -- it's rude to offer corrections without asking.
It does depend a little on what the mistake is. Like, if it's someone who clearly has a good grasp of English and typed "from" instead of "form" I might ask "(psst was that supposed to be 'form'?)". I've messaged people privately when it seemed like a word just got left out accidentally or something.
But if it's awkward grammar, or if there's kinda constant misspellings, I won't mention it unless the creator has asked for it, because that's a matter of correcting someone's writing, and not every creator wants that.. I think if you DO want that kind of correction, it's best to seek it out -- just leave a note in the comic's description or something that "English is not my native language, so if you see incorrect grammar or spelling, please feel free to point it out to me!"