Poker Face was bad I'll give you that. It's really the only song that completely jarred me, but in a Glee club there's bound to be times where the kids just want to sing to show Mr Schue what they can do. So I have no problem with those ''What the hell why are they singing this in front of the club?" songs.
And in the case of the Britney Episode, we know with the save Only Exception, that all the songs are sang by the kids as fantasies, Santana and Brittany are more shallow than Rachel and Artie, so the selections for them make less since (but then again I'm A Slave and Me Against the Music are all about dancing which Brit does love.)
Rachel is singing Baby about Finn, and Artie's is clearly about the breakup with Tina.