I'd say, since cats are known for being stealthy and sneaky, that Mercutio was calling Tybalt, well, stealthy and sneaky. I think the Prince of Cats thing is also an allusion because I remember reading about it somewhere but I can't remember much (eek! sorry...)
But yeah, I think he was saying that Tybalt is sneaky and stealthy, but also skillful in many ways, and treacherous, to say the least.
