It’s Simon Cowell’s 54th bday today … and he’s marking the occasion by cleaning the TOILET PAPER off his Beverly Hills mansion. 

So, who would have the balls to TP Simon’s pad? How ‘bout a group of 5 girls. 

The culprits are the ladies from Fifth Harmony, the girl band Simon signed to a record deal after they performed on “X Factor” last season. 

Besides the toilet paper, the gals also hung a banner that reads, “Happy 87th Birthday Simon!”

With friends like these …