Champ is a nickname for Pearl's friend Cambam010.
It originates from the Misadventures SMP, where players can talk to NPCs by typing in the in-game chat. There are certain NPCs that give quests that can give the player access to custom ridable mounts. One day, while Cambam was streaming, Pearl tried to talk to one of these NPCs, a rat called Champ, by saying "Hi Champ" in the in-game chat. Cambam thought that Pearl was talking to him, until Pearl followed up with the question: "How does one ride you?".
Pearl watched the clip of this moment live on stream[1], and since then Pearl's chat will usually erupt with "Hi Champ!" if Cambam shows up in one of Pearl's streams or in chat.