A Barb is a fan of rapper Nicki Minaj, similar to how Taylor Swift fans are called Swifties. If your teen says "I'm a Barb" they mean they belong to her fan community online. The name comes from Nicki's alter ego Barbie. It is harmless fandom slang. Sometimes Barbs get into heated arguments with other fan groups on Twitter or TikTok, but the word itself is not concerning. Capitalised when it refers to the fanbase.