When the Clonetroopers were working for the Jedi, they were dedicated, clever, and personable, whereas the Imperial Stormtroopers come off as bungling, uncoordinated drones with poor aim. Granted, as the original clones are presumably replaced by individual recruits over time, the people within the army will change. But is there any in-universe rationale given for the decline of the Grand Army's originally high standards?
The reason they became incompetence is because the clones made were firstly second generation clones which were based on dna obtained from the clones of the original jango fett so there is dna loss. secondly they were made in Spaarti cloning cylinders which is a 3 day cloning process, not allowing the clones appropriate time to be trained and have there cognition skills developed.