It's been around a few years now, so it's nothing new although people are still getting increasingly creative with the concept. Most people with taches have been ridiculed for having a tache (at some stage).
These days a lot of folks seem to love the tache. Anything with a tache stuck on it is automatically cooler, hip, fun and more sale-able.
I have to admit, i do own one moustachio item: a kitty I had to have for my studio. Other than that it's not a trend I follow. That said, I laugh at the moustache pictures and favourite the odd crazy moustache emblazoned Etsy goodie.
It's actually a bit of an Etsy in-joke: got something that won't sell? glue a moustache on it! I've even suggested combining trends: an owl with a moustache, wearing a striper cowl and chevron print waistcoat, with a cupcake hat and a steampunk necklace that looks about as steampunk as my 1990's jelly shoes. While this is just a big joke i have to wonder 'am I on to something?!' hehe.
I really would like to know why they are so popular. If memory serves Tom Selleck (especially as his Friends incarnation Richard) is one of the few who were ever adored for their moustache. That moustache even got it's own roles. Regardless of where it came from, it's been a pretty enduring trend and it isn't hurting anyone!