Specific Campaign

I am curious what are your thoughts on a campaign of DnD/PF that focuses solely on one type of monster? Like Undead and most monsters are undead and the villain either a lich powerful undead or a necromancer. Or perhaps a campaign focused on Feywild and monstrous plants? What you guys think is this being... close minded or some other word I can't think of or is this okay?

It's not close minded, it probably follows a theme, I had a game where they only fought vermin....and one druid, but mostly vermin, or the time I had them fight animals...or the time I had them fight Goblinoids

I prefer themes and would love to do a campaign focused on spooky monsters.

This sort of game, more than most kinds, will require buy-in from the players. If they're expecting varied opponents, or can and will specialize to deal with a certain type, the game may not go as you expect. Make sure everyone has the same expectations.