I started wondering about that since there is this guy called "blacklisted" who is currently a blue belt with 58% winrate. After I beat him he got salty and bragged that he already beat me on multiple alt accounts.

yes, they are allowed if you just play the game on it and not play in beginner lobbies or other places where u require to be a low belt

In addition and on that note, you cannot use your smurf to play against yourself. Multiclienting is strictly prohibited.

Alts are not a subject that toribash staff likes, but as said above they are allowed.

seems you got answered.