It's a fun game mode where people play random champions. The matchmaking is really flexible, so you might play with better players frequently.
So don't flame people for doing poorly, especially on champions they've never played. Its a game mode to have fun on, not a mode where everyone is playing their mains role/champion. If someone goes up against a d3 Caitlyn as a mid gold person first timing aurelion Sol and does poorly, oh well. It's really not a big deal. They're not a lesser person for it and they don't suck at league.
Sincerely: someone who is tired of seeing people getting flamed in ARAM