I think the only 5 powersets at a time is a fun gimmick, but the 50% thing is a double edged sword and I'm not sure what to think of it.
Take in point how one of his powersets was Beast. Now going by 60's humanoid Beast, his grab bag of powers include super strength (1 ton), super speed (40mph), then superhuman agility, reflexes, stamina, dexterity, and healing.
However, with Mimic's half-power stipulation, his strength gets reduced to 1,000lbs. and he's now slower than Usain Bolt, so are the rest of his abilities that he gets from Beast watered down to at least above average? Well, his healing should still be twice the speed of a normal human, anyway.
So while this stipulation prevents him from being godlike, which is good, it also means that he's almost forced to only copy the abilities of powerful mutants as to not be redundant.
This could work to his advantage though in some cases, as with mutants whose powers can be as detrimental to the user as to other people. Case in point, X-Man, who's power rivals the Dark Phoenix, but at the same time are also killing him.