I doubt coming “between” a popular ship in the fandom helps and that’s probably worth its own chapter in the book of how this whole thing went downhill.
But the short explanation for the hatred you often see for him would simply be: Racism. It’s not an accident that in spite of being an outwardly gentle person whose only crime is quietly asserting that he wanted honesty in a relationship (which? is a minimum requirement? to begin with?), people believe him to be aggressive and controlling. It’s not exactly subtle.
And the fact that people STILL call Danny a clone of Mickey seems to have very little do do with his actual character.
Bonus point for that being a complaint that started on the very day Danny Pink was announced as a character, with a picture of Samuel Anderson attached. Huh. I wonder why.