Considering he has no knockouts, TKOs, knockdowns, and has never seriously hurt/stunned anyone with the use of a kick (at least in the UFC), I'm going to say, no, his kicks really aren't underrated at all. He has a very slow, telegraphed wheel kick that I've never seen land clean and some push kicks/teeps he uses mainly to keep distance. I'd never seen him even throw a leg kick more than once or twice before the 2nd Diaz fight. The most notable thing he's done with his kicks in the octagon is contribute to gassing out Chad Mendes who was coming in on only a few weeks notice and obviously was not as conditioned as he could've been with a full training camp.
I will say that he USED to have a very tricky front leg hook kick that he used to catch people like Holloway off guard with back in his earlier UFC fights but that has basically completely vanished from his arsenal in the past few years.