Overrated: Derpy- I like Derpy, but there's just too much derpy stuff when I'm trying to look up some fan art of some other pony.
Vinyl- 3 seconds of screen time and has as big a fanbase as some of the Mane 6.
Fluttershy- Love Fluttershy, but there's sooooooo many flutter fans.
Underrated: Spike- Nuff said. Just Enough said! Celestia- She barely gets anything positive from the fandom. Molestia is.... awkward, and Trollestia is annoying.
Oh, and didn't I mention Spike? Yes I did. Spike is most underrated character on the show.