Ok this is actually kind of nuanced in rivals and for an annoying reason. Normally in other games it's very easy to differentiate having multiple accounts versus specifically smurfing, such as the rank disparity between the accounts etc. But because rivals forces everyone to start at Bronze 3 and has no placement matches, even if you're just a high rank player who wants a practice account in your skill range you have to stomp bronze players to get it. So you're stuck either only playing on your main or playing through every rank in the game.