No. It's "ok" because you're not screaming them to their face while they're on the ice trying to play. In fact, I'd bet very nearly 100% of the criticisms people level here are never seen, heard, or thought of by the players they're directed at.
You have a "right" to do many, many things that are just stupid and/or careless to do. Nobody is arguing that people aren't allowed to boo players. The question is, in my opinion - what are you trying to accomplish? And how on earth do you think booing is going to accomplish it? If you just want to make a stranger who isn't living up to your athletic standards feel bad, then boo away, because you will accomplish that task. If you want that player to be better, 9 times out of 10, booing is counterproductive, because people who respond well to heated criticism are few and far between.
And no, I don't yell or curse or boo the TV because it's a sports game being played by practical strangers miles and miles away. It's not worth the emotional investment IMO. I hope they win, yes. I'd like to see them succeed, yes. If they lose, they lose. I decided to watch, and I can almost always find some entertainment in the progression of players' and franchises' stories.
Also, you're not a buffoon by going to a game if you want to go to a game. You're staying home has done absolutely nothing to hurt Sather or Dolan or their bottom line. If you think it's worth staying home just to save yourself the money, that's fine, and I agree 99 times out of 100. But what on earth do you care about other people going to a game and enjoying themselves? "If everyone stayed home, we'd show them!" Yeah, if everyone did a lot of things we'd accomplish a lot of things. But they don't. And we don't. And such is life.
Bottom line, Nash hasn't been nearly good enough in the playoffs. He knows it. You can see it and hear it in everything he does and says. People can boo him if they want. If it's just about some catharsis for yourself, go wild I guess. I wouldn't, but everyone is free to decide for themselves. But, if you wanna light a fire under him, think about it for a second because it seems obvious to me that the fire he's trying to light under himself isn't going to get bigger as 20K fans boo him.