I don't really understand why everyone hates Piplup so much. The only problems I have with it are its somewhat unfitting dub voice (only dub Pokémon voice I don't like, btw) and the fact that it lost most of its sassiness after a while (which still appears now and again). It's not like we've never had a Pokémon outshine the others before.
I don't think its Piplup people hate, its just its overexposure. When DP first started Piplup was the same as Dawn's other Pokemon, it stayed inside its ball and Buneary/Pachi got equal screentime to it.
However sometime around Dawn's first Contest with Kenny, Piplup started becoming some major mascot out of nowhere and remained outside its ball from that point on.
Also, as you said, Piplup's personality has changed. Dawn/Piplup were very funny as a dynamic back in DP1-3....but since then they've just turned them into another Ash/Pikachu type of friendship, which is disappointing.
I guess its too much of a good thing. Piplup was awesome back in early DP but now everyone is mostly "meh" about it. Given that its highly unlikely it'll evolve, it also means we're stuck with it as is for the rest of DP.