That's an interesting point as accent isn't genetically determined.
Short of having Jango Fett actively present (which he was) 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 (like the Togrutan Jedi Master Shaak Ti, in addition to federation trainers) would have some influence on clone language.
As for Omega, remember that she is older than the Bad Batch. So if some records were substituted from Jango Fett to a mix and the result was a softening of the accent, that could account. Also, accents -- like language -- tend to drift over time. Either explanation would work.
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, the clone accent is like this. An oversimplification.
This might sound bad but really this sort of shortcutting keeps us from getting mired in all details of minor characters, backstory, and setting(s) when plot is what drives most all SciFi.
Addressing Valorum's question:
"accent isn't genetically determined" - Do you know this to be true in the Star Wars universe?
First, one can't prove a negative. It just leaves it open to things like "well, maybe Dave Filoni didn't say so but how do you know it isn't his notes. Oh, they just got published? How do you know they he didn't think that ..."
Secondly, notes two common races are Twi'leks (identified as a near-human species) and Humans. Humans are amongst the most common per Wikipedia and this very forum (SciFi & Fantasy answer).
Furthermore, and this comes straight from Wookipedia
Jango Fett was a famed Mandalorian human male bounty hunter and the
clone template of the Grand Army of the Republic.
In Humans, the accent isn't genetically determined. Otherwise, you would speak like the accent of your ancestors, regardless of being with them or not. And, that's just trivially false.
But, you know, a long time ago in a galaxy far, far away. In other words, it's fiction, and as smart as the writers were, I don't expect them to be linguists (or scientists) in addition to being writers. I'd go a bit into Socio-Linguistics which I think is the most cogent explanation of why all the clones speak the same but, honest to god, if we're staring from here, that's a bridge too far and I leave it as an exercise for the reader.