Yes. Many sources claims that the other "players" are bots.
I've noticed that playing tge same course over and over tends to repeat characters. It could also explain why you're able to choose 10 favourite characters on your player's profile. I think the "style of gameplay" gets somehow recorded and called whenever the game needs through AI.
Yeah, that makes sense. But the nicknames... Sometimes I wonder if they are bots or humans because of their choices. If they are indeed bots, who the hell came up with those weird names? Nintendo?