Unfair thing to measure him by in my opinion. Steve Ballmer inherited Microsoft when the stock traded at a P/E of 60. When he left it was 14, in line with many other tech stocks during that time. (Even Apple was lower)
He took earnings per share from $0.70 to $2.63 during his tenure. A CAGR of 10.5% across 13 years in one of the world's biggest companies. He couldn't control multiple compression following the dotcom bust.
By that metric, no he wasn't underrated.