I'm actually familiar with this as FL skeeters are annoying af. What they are doing is removing the ability for them to breed, so mutating is a moot point. The modified mosquitoes will mate passing along the dominant gene removing the ability for them to breed which will kill them off in a few generations (mosquito generations are cycles of a week or less I believe).
The real concern comes from disrupting the food chain because there are plenty of other critters that eat mosquitoes so removing one of their food sources could have potential to seriously mess with other species we don't intend to target.