Undertaker is good for a big man of wrestling, but don't pretend he never had shyt wrestling matches with other guys. It really depends on who he is working with guys like HBK, Punk, Angle, 
Triple H, are obviously talents who can work in the ring and understand ring psychology. 
It's hard to recall a bad match any of those guys have had, period. 
Undertaker will give his opponents a lot in the ring, so that makes for fantastic matches with his force of character and legacy.
When he's in there with some trash who doesn't know how to work it will look like trash. Plus, the man is almost 50 years old. After watching that match last night, if you think CM Punk didn't put in work then