I think he was rated exactly how he deserved to be. He was a brawler with knockout power, plus some boxing training and wrestling experience. He was a tournament runner-up at UFC 6 and Ultimate Ultimate '96, and was in the semi-finals of UFC 11 and Ultimat Ultimate '95. He hung with Oleg Taktarov for almost 18 minutes and world-class striker Maurice Smith for over 8 minutes, almost knocked out Don Frye, and went to decission vs. Severn (a much more decorated grappler) and Ferrozzo (a much larger fighter). Along the way he deteated veterans like Cal Worsham, UFC 3 winner Steve Jennum, a much larger Paul Varelans, Hugo Duarte, and "Cabbage Correira", being the only man to knock him out cold.
Obviously, due to his lack of training and conditioning, he's not going to measure up to today's athletes. But the fact that he had so little going for him actually makes his accomplishments a little more impressive. A K-1 semi-finalist should have been able to take care of Tank in record time, yet Smith took a relatively long time to finish him; a World Sambo champ should have been able to tap him within seconds, but it took Taktarov forever. Those kind of performances against the top guys of the time would place Abbott in the role of "gatekeeper" now, a position now held by the likes of Thiago Silva and Martin Kampmann, very good fighters in their own right.
I also think it's a bit unfair to criticize him for his commentary work. The guy's a fighter, not a talker. I wouldn't expect Mike Goldberg to be good at fighting, either.
I get that you don't like him, but it sounds more like a personal dislike rather than a critical assessment of his record and his skills. I've never been a fan, but in his prime (mid-late 90's) he DIDN'T "got the head kicked off him most of the time". He won more than he lost, and the guys he lost to were some of the best in the sport at the time. Not bad for an unathletic guy who hadn't wrestled in years and only hit the bag now and then.