Clone DNA degrades fairly rapidly due to the accelerated aging process, so it makes cloning from a clone less effective as that clone will experience DNA degradation exponentially faster. Even unaltered Boba had to deal with genetic degradation.

Also, after doing some research, it seems that I was wrong about the reason the Fett clones were phased out (or at least my knowledge on the subject was outdated), and that it had less to do with the loss of Jango Fett and more to do with the Kamino uprising and Palpatine losing faith in an army made up of only one man, feeling that an army made up of more than one donor would be less susceptible to being turned against him again.

The reason a lot of stormtroopers can't hit anything is because billions of them were produced using the Spaarti method (which Palpatine preferred). Due to the Spaarti cloning process, which condensed the growing time down to one year, it left only a few months for training, resulting in much less intelligent and capable clones, but with the ability to very quickly replace losses. And as we all know, Palpatine preferred quantity over quality (which is why TIE fighters don't have shields as it was cheaper to just replace a TIE than to keep one running apparently).

Also, the 501st didn't accept other clones or recruits, so they were still pure Fett.