There are definitely contestants I have rooted against because I haven't liked them. I had a friend who was up against James Holzhauer on the second taping day of his Jeopardy! run and supposedly he kept up the "Jeopardy! villain" kayfabe to the point where the contestant staff were complaining at him and this friend took an intense dislike to him as a person. I've softened since then, but it's a good example.
But mostly, I don't know contestants well enough to really dislike them enough to actively root against them. However, I absolutely pick favorites, even in person at the Price Is Right tapings that I've been to in the last year. I would root for anyone playing their regular game on stage but I caught myself holding back on giving advice from the bidders if they were competing with someone who I had spoken to in the waiting room before the show or someone who I knew through online TPIR/game show fandom.