From WSJ:

It seems Shino’s card in Boruto’s universe ninja card game is ranked as C-level ( = Common ), because he is not the famous/powerful type of shinobi. In fact, it seems the C-level is the most basic one.

This makes super easy for kids to find Shino’s card and that annoy them a little. In this short story, Kiba is walking Shino to his class when they hear about the whole card-game thing, and of course Kiba tries to hide his laughter (I guess he is higher ranked)

Other confirmed ranks:

R = Rare (e.g. Genin Naruto)
SR = Super Rare
SSR = Super-Super Rare (e.g. Hokage Naruto)