Because at times he's made faces and gestures on the court? Yeah, the, uh, "big ones" gesture is over the top but he's far from the first or only guy to do it. In fact it's originator (in the NBA at least) is Sam Cassell and he was a great PG mentor at the end of his career. A few other guys who have done that same move (now fineable by the NBA) are Kobe Bryant, Marco Bellinelli, Finals MVP Andre Iguodala, Kevin Martin and
probably a few others I'm forgetting.
Caron Butler has played 13 seasons in the NBA. He was rookie of the month 4 times his first year and on the all-rookie team. He is a two time All-Star, has played in nearly 900 career games and 64 playoff games including winning a ring with Dallas. He nearly threw his life away dealing drugs as a kid before turning things around for good - maybe I'm wrong but I don't recall a single off court issue in the NBA with him. He learned the importance of proper nutrition and taking care of his body. He learned to adapt his game as he got older and couldn't rely on his physical gifts.
Being a mentor doesn't mean you're necessarily someone who has always done the right things. Often it helps if you've done a lot of the wrong things, learned from them and are able to keep other guys from going down the same roads.
EDIT: Just saw this tweet from Jason Jones:
Jason Jones @mr_jasonjones 3h3 hours ago
Speaking to people in Vegas I've heard nothing but good things about how Caron Butler will help the Kings. Told he takes no BS from anyone