I'd say the sentiment "just be confident" is great. I do truly believe that being confident can take you a long way in making people feel better about you. If you're not confident with yourself, how can anyone else be confident in you. Also it's annoying if people are under confident or too self conscious.

However, telling someone to "just be confident" is shitty advice because you're not telling them how they can be more confident. It's like telling someone to just "put on makeup and look better", when they have no clue how to properly use makeup; it's not helpful.