I agree with Karnak's thoughts of, if you put planes on a scale from 1 to 72, that the P-40C is 72, something like a Spit 14 at the 1 end, and the P-38 near the middle, but in the upper 50%.
Triton puts it well, too, not inconsistent with that, and this is how I think of it, too:
It does a lot of things well enough to kill in lots of different ways, but it doesn't have very many (if any) 'crutch' attributes that the pilot can count on to pull him out of a jam. The easier planes in the game all have one or more outstanding traits they can fall back on to get them out of trouble.
With regard to the Mossie, I've flown that around recently and am not very good in it. I seem to get taken out frequently by pilot wounds from a sprinkling of MG rounds that in most other planes would have been inconsequential. There are some folks who are very good in it, of course, but I don't think of it as an easy plane to fight in.
I suspect that popularity in the MA correlates somewhat with how good the plane is in the MA.