I see we're all using "underused" and "underrated" interchangeably. No biggie; it's all opinion-based anyway.
IMO, DiBiase was both underrated AND underutilized.
He was trained by the Funks down Texas and was being groomed to become the next NWA world champion. That fate changed when the old man lured him to the Fed.
Ted was given the gimmick Vince had dreamt up for himself when he was a kid: a cocky millionaire who bought his way out of everything. The character was a perfect heel, and Ted portrayed it excellently.
The biggest criticism of Ted seems to be his ring style. While his mic work and vignettes perfectly suited the gimmick, Ted's ring work was often not "dirty" enough. He kept everything tight and it looked very good and technically sound, but he maybe should have adopted more sleazy tactics. He used some in his matches, but many folks feel he relied too heavily on his mic work and Virgil to get over as a heel.
That said, he got a decent push, but I always felt they should have put the big belt on him at some point. According to some sources, DiBiase was originally in line to win the strap in the tournament at WM IV. He had tremendous heat and good momentum going at this time, and it was a shame to see him almost relegated to mid-card status following WM V, after which time he would engage in mundane feuds with his manservant Virgil (who is very much a prick in real life) and a morbidly obesce albino hermaphrodite with a peroxided perm and raccoon eyes named Dusty Rhodes.
Ted would again gain steam teaming with Mike Rotundo to form Money Inc. They would become the WWF tag champions, which would be Ted's only "legitimate" title reign in the Fed. They worked a memorable program with Hogan and his manservant, Brutus Beefcake. Ted sort of faded into the background once again following the Money Inc. run, eventually ending up in WCW, where he wound up managing the Steiners before becoming one of 83 members in the nWo.
Considering some of the guys they did push, I always thought it a shame they didn't do more with Ted.
In my book, he was underrated and underused by management. At least the fans know better.