There has been skilled HWs in MMA before. We are just in a terrible era for HW mma where the average age of the division is like 70 because for some reason theres not enough new talent coming through to retire the old guys. But if you go back to around 2011 I actually think HW MMA was briefly on par with or stronger than HW boxing. Pre-USADA, Strikeforce HW GP, prime Cain JDS and Brock in the UFC.
Its obviously not even close now though. HW boxing is vastly, vastly superior to HW MMA at the moment. In terms of athleticism, skill, depth, prospects....everything, basically.