Answers will be all over the place because there's not actually a legitimate MMA HOF, so everyone is just making up their own criteria.
In my opinion, I'd say he belongs in the HOF. The guy has the second most wins in UFC history; has the second most submissions in UFC history; and has consistently been a top 10 fighter (in 2 divisions) for more than a decade, with 2 title shots. That seems HOF-worthy to me.