I don't think Nidalee cougar cool down was the problem.

I think the problem is that how her cougar form does not even cost any mana to spam at all.....

If you want to nerf her then don't nerf her cool down since that won't do anything at all when she starts to get cool down reduction but give her Mana cost for cougar even if it's little just make it 15-20 mana for them so she doesn't get to abuse her broken damage and escape if she doesn't land her spear.