I think George McGinnis underperforms for his ratings and maybe that's because he's been misused but a forward thats B C C+ B+ B+ should be doing more than 10/12 with 0.8bpg and 2.6TOPG. Also his splits suck.
Hardrick came to my mind as well as others have mentioned outperforming, his stats were impressive compared to ratings.
The one's who often under perform I feel like can often have a big flaw such as TO's. Like Ja had good ratings didn't he? But that killed it and made him a negative player.
Moussa Diabete when I had him had like B-/B rebounding when I had him, but his Per 36 numbers would always show that he was rebounding at a rate that you would expect out of an A- rebounder
Peja definitely comes to mind as a mega under performer