I'm pretty sure Delphox is a popular Pokemon...I've barely seen any hate until just now when I had to go look. Here's all I could find and some things I heard before:
-A lot of people are annoyed about how it looks feminine but still has a 87.5% chance of being male.
-Another thing is the huge ear fluff it has.
-Some people also dislike how Delphox is a fox, as they're one of the most popular animals out there.
-A lot of people were annoyed about Delphox being bipedal.
-When people heard about it being like a mage, they expected something "cooler looking"...or just not a fox with a stick wearing a robe.
-Some people who like Braixen were disappointed when the evolution wasn't as cute.
-The in-game sprite/model apparently looks weird?
-It's "the worst of the three Kalos starters".
-Decent Special Attack and Speed but not really anything else.
That's pretty much all I've seen. If there are any reasons someone else has for disliking Delphox that aren't listed here, feel free to tell me and I'll add them in.