For me, I think some of Piggy's worst performances were the majority of her appearances in the first half of The Muppet Show's third season (between the Kris & Rita and Cheryl Ladd episodes). Why? Because I think that she was the most mean at that time. Her actions in the Leo Sayer and Marisa Berenson episodes especially bother me and come across as VERY mean-spirited to me. Even the Piggy puppet from that period bothers me, because that particular puppet of her looks REALLY p*ssed-off and makes even the Season 1 Piggy puppet look gorgeous by comparison.
A smaller one is her violent actions towards Robin in the Vet's Hospital sketch in the Linda Lavin episode. It's one thing to inflict that kind of physical harm on the adult characters, but not so much on a child character.
And, of course, most of the guest appearances, interviews, etc. that showcase the Flanderized, one-dimensional, karate-overusing "what did you say? HI-YAAH!" version of her that have become way too frequent ever since Frank stopped performing Piggy.
At least the 2011 film pretty much restored the pig's original "Jerk with a Heart of Gold" personality and hope that it's the beginning of them gradually restoring the pig's original complexities, hoping that the 2014 movie will continue down that path.