You’ll have to forgive me for not knowing exactly how to tag this song. In one sense it’s a mashup; in the other sense, it’s a cover. Why the confusion, you ask?
Well, the song takes its vocals from Lana Del Rey’s “Video Games,” though Monarchy is singing the piece. Where it gets confusing is that the backing instrumentals are from Monarchy’s original song “Phoenix Alive” — which did well in and of itself.
So, in a nutshell you have a cover of LDR’s “Video Games” that overlays her song with another original song. Thusly: mashup vs. cover.
In the end, I’ll give credit to LDR for having the original song. Rational being that: 1) it’s probably the song you recognize; and 2) by definition, it’s quite typical for a cover to maintain the original vocals, but change the instrumentation (doing the opposite would likely result in a song not recognizable as the original being covered). And in this case, LDR’s vocals come out on top.
Whether you agree with my interpretation or not, hopefully we can align on the fact that this is a good song.