It's a five cost terminal silver (silver clogs decks, this has already been pointed out) that isn't as good at clogging the opponent's deck as Ambassador, Sea Hag, Witch, Mountebank, or anything else and isn't as good at helping your deck as a Laboratory, Vault, or Trading Post. Jester is probably in the lower half of five cost cards in my experience. Ambassador ALWAYS trashes from your deck and clogs theirs, Torturer ALWAYS at least draws you three and threatens discard or curse for most of the game, Mountebank gives twice as hard as anything else and trades missing sometimes for that honor. Jester might help you or hinder them or it might do just about nothing. Against decks with high density of good cards it's better than usual, however if your opponent already has a lot of good cards it's probably too late for you to steal anything that matters and draw/play it enough to count for anything. Against a big money player with a warehouse or two it's mediocre at best.
As far as how to counter it? Most of the time you won't counter any attack, you'll simply play around them and win or mirror their strategy and simply make fewer mistakes. This is no different. I would certainly rather get Jestered than hit by any other non Rabble five cost attack.
Watch Jester ride the middle/bottom of the pack.
Also, feel free to jump over to the popular buys on CR and see Jester with a .99 Win Rate With and 1.02 Win Rate Without.