That's an interesting point as accent isn't genetically determined.
Short of having Jango Fett actively present when the first few batches of clones were learning language and then having the clones pass it from generation to generation as a matter of group identity, one would think they would sound more like the Kaminoans who made them. Even more likely would be that clones would have a mix of influences from others (presumably Jedi and federation trainers) would have some influence on clone language.
But all this sort of shows a weakness in most SciFi. Namely that there is sort of a broad brushing of things. For example: It was raining on Alderan (Wha?!? The whole gotdamn plannit? At the same time?) Another example is how homogenous entire planets are ("But Alderaan is peaceful! We have no weapons, you can't possibly...") perhaps with a touch of something the opposite to show it's diverse and realistic in some token aspect. Unfortunately, clone accent is like this. An oversimplification.
This might sound bad but really this sort of shortcutting what keeps us from getting mired in all details of very minor characters, backstory, and setting(s) when plot is what is driving most all of SciFi.