It depends on why your girlfriend is unhappy. If you’re worried about her, you could try asking about it, and whether there’s anything you can do to help. But pay attention to the response you get, because your help might not be what’s needed. Also take a moment to think about your motivation: are you asking because your girlfriend being happier would make you more comfortable? If it’s actually about how you feel rather than how she feels, that’s a totally different conversation and you should have them separately.