I think a decent analogy is that Kobe is Derek Jeter. A legitimate hall of famer but shouldn't be in the discussion for one of the best players of all time.
The stuff is not clearly there. His curveball is very inconsistent and doesn't fool batters often. He's a two pitch guy. Just because he throws hard doesn't mean he has ace potential. Joe Kelly throws harder and has a better breaking ball. Do you think he's a potential ace?
Martin is fifth in the majors in terms of innings caught, but also over 50 innings behind the leaders in Derek Norris and Yadier Molina. Not saying he shouldn't be catching more, just giving the info.