It's not even just mumble rappers, they came after the mainstream hip-hop rappers were glamorising it first. Rappers like Lil Wayne, Waka Flocka, and Future were rapping about Xanax way before mumble rap became a trend.
This is from 7 years ago:
Lyrics:
Pay for what girl you better pay for this dick
She the type to fall for two Xans and an outfit
I’m with the Xan crew so it’s hundreds on the floor
My hoes is off in the mo, I be balling ho
(Chorus)
Rooster in my Rari, rooster in my Rari, rooster in my Rari
Okay, flex!
Rooster in my Rari, rooster in my Rari, rooster in my Rari
Okay, flex!
These hoes, we done ran through them
Xans we done chewed them, lames better salute them
F-f-flex!
These hoes, we done ran through them
Xans we done chewed them, lames better salute them
Squad! Squad! Squad!
(...)
Fuck her all night and never call her back
Homerun the pussy every time I swing my bat
Squad!
And if it’s good I’mma double-back
Desperate need, I need some Xans! Where my doctor at?
If it’s good I’ma double-back
Desperate need for them Xans where my doctor at?