She was both an over and underachiever. When she first emerged nobody had her winning 7 slams, being one of the 3 dominant players of the last 15 years along with the 2 Williams, and being one of the 3 top clay courters of all time along with Graf and Evert. She far exceeded all expectations. She was also an underachiever quitting like a coward at 25 due to a mini slump, and just presuming with her playing style she could come back on cue and dominate again (far from the case as we saw), and since she had that serious viral infection for 2 years in the middle of her prime.
Definitely should have won atleast 1 Wimbledon, and would have smashed Evert's all time RG record had she kept playing without a retirement until atleast 30. However also was lucky to play in a weak clay era, and peak and win all those big hard court titles when Serena was mostly AWOL.
As for compared to Graf some aspects of her game are better than Graf. Much better backhand, better at the net, possibly better return of serve. However Graf and her huge weapons in her serve, forehand, and athleticsm, still probably would be more effective than Henin's all around game.