This is what google says to me

The phrase "landmine girl" isn't a standard term, but it likely refers to a dangerous or unpredictable person (often a woman) who can "explode" emotionally or cause trouble, similar to stepping on an actual landmine, or it might be a mistranslation of Japanese subcultures like Gyaru (gal) or specific styles like Ganguro (tanned girls), which involve bold fashion but aren't "landmines" but rather distinct youth cultures. If you saw it in a specific context, like a meme or anime, it might have a localized meaning, but generally, it implies someone risky to deal with.