It means someone who is uninteresting, often due to only enjoying mainstream hobbies and interests.
[…] 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.