I voted Yoshi in the poll, but really, the thing that bugs me about Yoshi are those friggin' sounds Nintendo keeps recycling. They came from Yoshi's Story, a not very popular N64 game that starred Yoshi (*surprise!*).
Anyway, Yoshi would be better if they stopped using those YS sounds and went back to the sounds used in all pre YS games..
As for other characters that I hate..
Baby Mario/Luigi- they mess with continuuity. I know Miyamoto has said that he doesn't care to tie characters to a timeline, but when baby characters can appear in the same game at the same time with their grown up counter parts, there's a problem.
Baby Bowser/Bowser Jr- this is just laziness on Nintendo's part. They are just using the Baby Bowser (or Koopa Kid, which they are not) model from Mario Party 4, adding a bib/mask, and calling it Bowser Jr. I dislike everything about this abomination. What happened to the REAL Koopa Kids (Larry, Morton Jr, Wendy O, Iggy, Roy, Lemmy, Ludwig Von)?
Petey Pirahna- This is the wrong way to re-do a classic enemy: turning it into a giant baby Pirahna Plant. Petey should just jump into the fires of Corona Mountain, and take Bowser Jr/Baby Bowser with him. It would do the entire Mario-verse good.
Piantas/Nokis- I really don't dislike these paricular characters, I just wanted to say that I don't like how locations and characters from SUper Mario Sunshine are being used and reused. Super Mario Sunshine was good, but it wasn't that good (imo).