No, she isn't happy. Her life is mediocre. She's "on hold". Worse: she's drowning, and the process of struggling back up to the surface is hard. Especially with all those hands trying to hold her down...
I suppose I should say "spoiler alert", if you're sensitive, but in my opinion, the more you know about "Joy" going in, the more likely you are to enjoy it. This is a (basically) true story about a woman who puts her life on hold to take care of her dysfunctional family, and how she eventually shakes herself awake and becomes a success, on her terms. (If you want real spoilers, look up Joy Mangano's bio in Wikipedia). Based on that:
...it's starts slow, just like she does. At least the first half of the movies is pretty much a downer.
...you spend most of the movie with her family, and they are mostly an unpleasant lot: codependent, manipulative users. So are most everyone she interacts with in business. She's got one good friend, and her kid is cool. Other than that... expect to dislike most everybody.
...Success does not come easy, so expect a lot of disappointment on the way to the ending, which IS very satisfying. Maybe all the more so, because of what you (she) goes through to get there.
I was disappointed when I watched "Joy", because David O. Russell has shown how much fun he was able to find in war ("Three Kings") and mental illness ("Silver Linings Playbook"). Here, he choose not to do that. He could have made this fun, but chose not to. Instead, it's frustrating, and that ends up being inspirational: after watching this, I really felt a charge, a push to get over my BS and make things happen in my life. And that's a good thing.