From what I know of Santa Muerte, (Heard from my Abuela), She is NOT a Catholic saint. She's more of a Mexican folk saint, worshipped primarily by farmers and members of the cartel.
The Catholic church actually regards Santa Muerte's following as more of a cult, in that, because she is not a "saint", and has no actual relation to the Catholic church, her followers are "worshipping a false idol", which is generally frowned upon in Catholicism, and Christianity in general.
That is mostly correct Minerva. Santisima Muerte, The Most Holy Death, is a Mexican folk saint of death. She tends to be worshiped by lgbt and criminals because she is accepting of all no matter what; she does not judge. She is a psychopomp in a way and protects her followers and ensures a safe passing into the afterlife. She is death and can teach us lessons from death. That's a basic view. I suggest checking out a youtube channel called owlglass30, he has a lot of good informational videos on her.