No he's not. When you're that small compared to the average fighter, physics work significantly in your favor due to the square-cube law (
https://en.wikipedia.org/wiki/Square-cube_law#Biomechanics), and your biomechanics is fundamentally different from normal-sized fighters.
That's why cats can fall 5 stories and land on their feet uninjured. That's why if you look at any "P4P" list of fastest, strongest, highest jumping etc. animals, you'll find that they're dominated by insects, and the gap is ridiculous. That doesn't mean they're better at any thing, it just means that physics work differently for them.
Mites for example, can have an equivalent speed (relative to normal sized human) of 1,300 mph. Are they better runners than cheetahs? No, they're just tiny and the physics for tiny things are way different, that's all.
Mighty Mouse would get destroyed by a bigger guy that "looks" less skilful but is in actuality a much better fighter than he is, just like cats would get destroyed by tigers even though "P4P" their stats are way higher.
In reality, Mighty Mouse is one of, if not the least skilled male champion in the UFC, for the simple fact that every other champion would most likely beat his ass in a fight.