It means someone who is uninteresting, often due to only enjoying mainstream hobbies and interests.
Some choice quotes from the Urban Dictionary:
[…] A person who has failed to develop their own personal tastes and therefore has adopted boring, conformist, tastes
[…] A person that follows the crowd and copies other people who are more well known/ popular than them. A sheep.
[…] There is nothing standout about this person, as they like whatever is “cool” and mainstream at the time be it fashion, music, TV shows, etc. This dull, shell of a person participates in “group think” even as an adult. A Basic Bitch most likely wears leggings as pants, loves Starbucks, and watches shows like “Keeping Up With The Kardashians” and “Real Housewives.” A Basic Bro (who is also a Bitch) can be seen sporting the bro-uniform blue button downs, “loves sports,” listens to Country (because it’s “cool” now), and reads Maxim.
Why I Looked It Up
I had heard it for years, and knew it was an insult, but never quite knew what it meant. In The Good Place, Eleanor’s favorite insult was “Ya basic!” (insert finger snap here).