just want to know why his mouth is always covered

It was a gimmick that after a few shows became a habit; in fact, I read once that producers had to work really hard to come up with all the different ways to hide the bottom half of his face, like a challenge.

always wondered that myself

Just for laughs.

His entire face wasn't revealed (on air) until after the last episode when they all came out on stage for their curtain call.