There has to be a story behind this. I don't know the story, so I will tell you what I can observe.
Lofi hip hop radio is a channel on YouTube featuring a mix of mostly instrumental hip-hop music. Being a YouTube channel, it also has a video:a loop of around ten seconds, showing a young woman studying, taking notes in a book while her cat sits in the background, its tail swishing. She is wearing headphones, perhaps to suggest that she is listening to the same music as we are.
The music comes from a genre that seems to have been codified by this channel itself. The term "hip hop" might be a little misleading, since there are no record scratches, very little boom, and even less bap, and usually only rapping only comes in short, sampled fragments. This is more like ambient music, than anything else. Although, as promised, it is very chill, and is good for background noise during study or relaxation. The entire channel exudes chillness.
And rather than being just a niche thing, this channel became somewhat of a fad, a meme even, and has a number of adopters and imitators, with the same style of music and various chilled-out anime mascots. Something about spacey electronic music seems to resonate, apparently.
For me, it is also amusing how the denotations of a musical form change over time. "Hip hop", which has been taken as either a humorous or threatening musical form during its long history, is now a type of smooth jazz with an adorably studious girl as a mascot, and is meant to promote good studying habits.