Down bad means desperately into someone romantically, often to embarrassing extremes. "He's down bad for her" = he is humiliatingly obsessed. Originally Black American slang meaning generally low or struggling, now mostly about romantic desperation online. Not concerning unless it describes obsessive or stalker-like behaviour. Mostly used jokingly between friends to roast someone with a crush. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.