There's an interesting aspect of reading any kind of dialogue in that sometimes you have to conciously consider the accent or dialect that the character is using. This applying to scripts, novels, comics, anything you're not actually hearing.
Sometimes it doesn't matter and things can just flow, doesn't matter if they have a British or American or Australian accent. Sometimes authors have such a light touch and an ear for dialogue that they can place an accent without using use blinking neon signs to do so. Of course, for a reader to pick up on really artful cues they need to already be able to detect the flags - that is, they need to be already familiar with the vocab and so on of whatever dialect is being depicted.
Sometimes (and mostly I'm thinking of Gambit here, whenever he shows up) you're positively bludgeoned around the head with the accent. In fact I can think of many, many examples from the X-Men, including Emma Frost, who we know is posh and English and theatrical because she uses words like "darling" and "beastly"; Rogue, who we know is from the South because she says "y'all"; and my favourite, Phantomex, who was putting on a fake French accent. I'm still not sure how well that worked in a comic book, but the idea was lovely.
But sometimes you have to actively consider what kind of voice the character is speaking in. I love Ultimate Iron Man as Warren Ellis writes him, but I keep having to remind myself that he's a louche American, because in my head his dialogue sounds just like Terry Thomas.
Then again, if you can use comic books to have your cake and eat it too by having Iron Man as an alcoholic, US playboy with droll, British wit... why not just do it?