I received a summons to small claims for a frivolous lawsuit. I know that not showing up is a reason for a default judgement to be issued against me. But if the plaintiff will have to show proof to his claim, I'm certain he won't be able to. What is the law/practive in NYC small claims? Does the plaintiff have to show some sort of proof? If yes, I won't bother to appear.

Yes, the Plaintiff has to prove up his case whether you appear or not. Of course, if you're not there it's a lesser burden of proof.

How do you know the Plaintiff isn't going to show up with some sort of fake evidence?

Of course the plaintiff has to bring some sort of proof for the Judge to look at. You not showing up means essentially that you are not interested in the proceedings or do not contest the proceedings and decided to be a no show. If you are comfortable with someone getting a judgment against you, don't bother to show up as they will get that judgment and you will then have to pay it.