I'd say that he is, really, but a pretty good one at that. He's only had 1 titl shot by the way, Velasquez was not champion during their first fight.
He was thrown at Velasquez twice, first on the strength of his win over an out-of-prime Emilianenko and because they needed to boost Velasquez's profile after being knocked out in the first round by Dos Santos, secondly after knocking out a comically overrated Overeem WHO was being groomed to be the top challenger for the title. The outcome of both Velasquez bouts was predictable.
Nobody actually expected him NOT to get smoked, so he was never considered a genuine contender. Bigfoot will never see UFC gold.
On the other hand, he IS a 12-foot tall overachieving upsetting machine, stopping both Emilianenko and Overeem when everybody expected them to walk through him. For that reason alone, he's worth keeping around and has definitely made his mark on heavyweight MMA history. I still think he should be fired for getting caught cheating twice though.
Yeah, Shawarma basically said what I was getting at. The UFC has continued to feed him to rising heavyweights who need a win over a top opponent. Sometimes he wins despite no one expecting him to (like against Overeem), which is what got him that title shot despite getting smoked by Velasquez only a year prior (in a match that was given to Velasquez so a rematch with JDS would be justified).
I'm not taking away from Bigfoot as a fighter, just making on observation on what the UFC likes to do with him when it comes to matchmaking.