Bob Ross (1942-1995, 2013) was an American artist. He was born in Florida and got his inspiration to be an artist in Alaska. He painted a picture in a half an hour using a technique called "wet-on-wet".
When Johnny, Elliot and Matt played Mario Kart Wii (which was 18 years after Bob Ross died), Matt was replaced by Bob who played as his own Mii. It was never revealed where Matt was during that video.