YES, it's not real-life accurate... and it's not supposed to be. It's exaggerated, just like a lot of things in the game-setting, to help add to the over-the-top cliche-soaked "comic-book superhero world" feel of things.
Seriously, though, why is everyone hung up on Tracer's Cockney accent, and nothing said about Rheinhardt's German one, or Hanzo's Japanese one, when those accents are just as unrealistic as hers? It's all part of the game flavor.
Heck, that gunslinger cowboy character - I just bet once they reveal him, he'll sound like every gritty Wild West loner to ever draw pistols at high-noon.
Frankly, I'd be disappointed if they didn't make him sound like all the best parts of John Wayne and Clint Eastwood mixed together, considering his visual design is so stereotypically "The Good The Bad And The Ugly" I almost expect to start hearing the theme song start up when I look at him.
Exaggeration, unrealism, it's all just another aspect of the superheroism "excite your inner six-year-old" concept the game is based on. Tracer doesn't sound like a spot-on native Londoner, no more than Widowmaker sounds like she grew up in Paris or Torbjorn sounds like he's fresh off the jet from Stockholm. It's like the TF2 classes - the Demoman is exaggerated Scottish, the Heavy's exaggerated Russian, the Engineer exaggerated Texan, etc.
Stop complaining and just let it go - none of them are meant to be realistic, so please stop whining that they aren't.
MOD EDIT: