Problem is that as you get farther into Plus, def/mnd steadily matter less and less against most kinds of attacks. It never goes completely out of style, but it's really not the focus. It's damage reduction passives and max HP. (Which is boosted -so- much on base level, plus if you use Eirin, well...) When most of your party is awakened, a lot of attackers have some pretty intense damage reduction abilities, and more than a couple who just plain resist dying.
At that point, having nice def/mnd and above average base hp isn't quite as meaningful as before, and she only gets direct damage reduction if she spams her Physical attack. Not when you've got people like Youmu/Mokou/Akyuu who literally won't die, Meiling with ~42% direct reduction, all the people with non-dependent Fighting Spirit, etc. And all of those people can deal pretty intense damage. People who -don't- carry that level of durability mostly feature either even MORE damage (marisa spark with murakumo boost) or some kind of specialty, like ignoring defense/mind (REALLY important on some very hard fights) or having support stuff on the side.
Kokoro's got a side-support! ...but the masks only effect stats by like 10%, so it's a really weak support.