Low-key = not obvious OR not a popular opinion (this isn’t a bad thing and is the speaker’s opinon)
She’s low-key cute. —> People don’t usually say she is cute, but this person think’s she is./ It’s not obvious that she’s cute, but she is
I low-key hate this song —> It may seem that I like this song, but I actually don’t / People think this song is good, but I don’t really like it.
High-key = obvious OR a popular opinion (FROM THE SPEAKER’S POINT OF VIEW. It can be an unpopular opinion)
She is high-key gorgeous as fuck —> She is so beautiful (it’s obvious).
I high-key love pistachio ice cream —> I really like pistachio ice cream
High-key and Low-key are used to show the SPEAKER’s opinion
Example. They may high-key (really) think apples taste the best, but that doesn’t mean everyone agrees.