Tracer's accent is overdone on purpose. None of the accents in the game are really that accurate, and they're not supposed to be. The game's art style is cartoonish and exaggerated and so are the characters' voices. If the game had a photorealistic design then I could understand some of the complaints, but I think people just need to calm down and realize that it's all supposed to be a bit goofy and parodied. PS. I'm from Boston so I know how it feels to have my accent mangled :)
blehh tracers just kinda stands out to me, but yeah i get it. and ppl probably mostly notice their own accent being destroyed so yeah.