Is Mindfulness good for mood & stress?

Yes — the evidence for this is strong. The clearest and most rigorously tested application of structured mindfulness programs is relapse prevention in recovered depression: an individual-patient-data meta-analysis in JAMA Psychiatry, later corroborated by a network meta-analysis, found mindfulness-based cognitive therapy (MBCT) reduces depressive relapse about as effectively as maintenance antidepressants. That's reflected in an evidence score of 79/100 for its role in mood & stress, drawn from 11 studies (2 RCTs), with an estimated impact on longevity rated moderate (66/100).