It's also interesting to note that the mask sales man clearly vanishes into thin air at the end of Majoras Mask, it kinda makes me wonder, if he's that powerful why doesn't he attempt to take back the mask himself?
https://www.youtube.com/watch?v=taB0_lKtvoI
at 4:20
Well, I could think of a few reasons.
One of the reasons being, he had this all planned out from the start, and the reason he did all of this and made Link do the work is because he wanted to help Link. Help him grow with the idea of loss, hardship, and learn about all the people he comes across. He
does tell Link "Whenever there is a meeting, a parting shall follow. But that parting needs not last forever." And what he teaches Link in Majora's Mask goes over to Twilight Princess and helps Link teach the new Link and save himself as he does.
Two, as powerful as the Happy Mask Salesman is, he can't take on the mask itself as it might overpower him. All he can do is keep resetting the clock and stall time.
Three, this one is going more by the Manga itself. The Happy Mask Salesman was never a good guy, but a bad person doing good things for the sake of his own goals. Likely sending Link out to do the dirty work and play victim as he does, only to at the very end get back Majora's Mask, which may or may not have actually been 'cleansed of evil'.