Cfrock
Keep it strong
I read a book once where a dead guy who had unfinished business built himself a robot body so he could touch things, and so people could see him. The mask salesman could probably pull that off, he strikes me as someone who could be a skilled crafter. Maybe this robot body is only visible to the one who can help get the mask back(Link). That would explain why nobody in clock town mentions some weirdo hanging out in the clock tower with a backpack...
The idea of him being a ghost is unlikely for reasons already stated several times (Happy Mask Salesman doesn't fit with any established form of afterlife in the Zelda Universe) but the idea that he is a robot that only Link can see is so far-fetched and unlikely that I would like to say, drugs are bad and you shouldn't do drugs because... Because drugs are bad.
I'm only joking, I don't mean to insinuate that you do drugs, but my point is there isn't any reason to suspect that the Happy Mask Salesman is a robot and compared to that, him being a ghost seems almost likely if you ignore that it doesn't fit with established lore.