Well, in a battle that's difficult for all four of Mii to deal with, Cool seems to good because it can dodge to serious attack.
Conversely, when three low-level Mii and one high-level Mii go out (mainly to raise level), it is quite good to have a high-level Mii personality (unless the low-level Mii is weak like level 1, it is quite good that high-level Mii be attacked instead of the low-level Mii).
P.S. But the situation I said behind the scenes is likely to happen only after the ending, so in the early stages of the game, Cool would be good as I said earlier.
*my speak may sound weird because I'm using a translator.