Tache is slang for a loser (a dud), not necessarily a bad or dishonest person but a derogatory term nonetheless. It’s very modern slang, I have a 20th c. slang dictionary and it isn’t in it.

I don’t know for sure where it comes from. Most likely from the colloquial phrase faire tache which means “to stand out in a conspicuous or ridiculous way”.

Un monde de taches → a world of losers.

Indeed, “une tache” is primarily a spot, a smudge.

By extension, it is a slang word to designate metaphorically someone who “does not fit correctly” : incompetent, inept.

It is used in France since the 90’s approximatively.