Sometimes it does help, but, imo, the point of mindfulness is not to be happy, it's in the definition itself: to simply be more aware. I don't feel happy when I'm aware of my misery, I just am aware I'm miserable.
That awareness can often lead me to a solution because I wasn't even aware of the problem until I sought awareness. I'll be kicking around the house pissed off, not even aware of it, and then do a dumb mindfullness activity, and thus the annoyance is brought full into my conscious attention, "oh right I'm pissed off right now. Why? Shit, I forgot to eat breakfast, I'm hungry." or "I didn't walk today, that probably isn't helping," or, "I'm stressed about the three tickets I gotta get done, lemme just write them out as a TODO to at least externalize the stress." And then I can feel a bit better.
Or, not, I'll be like "I'm too pissed off to even think about why I'm pissed off." At least I'm not a slave to my emotions then, which is an important value to me: control. Even if I'm going to be angry or sad or whatever regardless, I like the additional awareness.