Athleticism is context specific.
Being able to lift heavy shit off the ground/from your chest/above your head requires athleticism.
Being able to run 42km at a constant pace requires athleticism.
Boxing and kickboxing requires athleticism.
Grappling requires athleticism.
None of those things use the same primary athletic attributes as the others. MMA is the weird one, in that you need the speed, endurance and explosive elements of stand up (primarily aerobic) with the muscular endurance, strength, balance and agility of grappling (primarily anaerobic). Both have some cross overs, but one is more of an aerobic exercise and the other is mainly anaerobic.
Being too muscular has its disadvantages and the same with lacking muscle mass. But then it depends on your style. Gumby didn't need muscles.