Well… that is not entirely true and the notion of what “happy” means is not properly broken down in a hasted fuzzy statement such as this one. “Happy” has become one of those words where all sorts of things are pushed within its boundaries. When people say they want to be happy or that this is the highest goal they strive for it usually means “I want to experience more positive than negative emotions in this existence and the way I’ve set up my life here”. But this does not necessarily mean “happy all the time”. You might feel overwhelmingly positive and extract meaning from a difficult thing you’re doing like rising a child. You may not be blissed out while you’re doing some or most of it due to the gravity of the task but you’re still fulfilled by what it stands for and the role you play within it. So is this to be denoted as the opposite of happy because it has all sorts of challenges? As unhappy? No. “Happy” has some wear and tear on it from over usage in naive positive philosophies and movements which don’t take into consideration that life is in its essence sometimes difficult and that this will be entirely unavoidable if you aim to do anything worthwhile with the one you’ve been given. So “happy” is a rest stop depending on the constellation of circumstance but deep joy and fulfillment are something entirely different – a decision.
Are “happy” people really healthier? Since the naive happiness cannot be held for a longer periods of time without in some way being delusional or (intentionally or unintentionally) blind, it has no long-term effect on wellbeing. What does have this effects and results in better health is learning how to control your emotions to keep your stress hormone (cortisol) levels in a normal range, because long exposure to cortisol floods results in compromised immune system, organ dysfunction, inclinations towards obesity, depression, anxiety, cardiovascular issues… Lack of deciding to deal with things is what makes for poorer health and physical manifestations of stress exposure. “Happy” has nothing to do with it because it is unsustainable without ignoring the reality of life in some way.
So are happy people more creative? Yes, for a brief period, and then the personality takes over. Psychological evidence leans towards exactly the polar opposite actually, meaning that some of the most important traits of creative people are derived from disagreeableness – the unwillingness to take things at face value just because they’ve been told that’s how it’s always been done but they find new innovative ways to view the world. The second trait is called openness and is very much entangled with interest in ideas and highly correlated with intelligence and fluidity of though. This is a character trait unrelated to the transient moment of “happy” and, as hard times almost as a rule have more lessons to teach us, creative people might even be more creative when they are not too comfortable but are exploring and scraping their knees in a way that becomes beneficial to the further development of ideas by influx of new information that so often comes in form of hard lessons thorough not so “happy times”.
So no, creativity and health are not that strongly tied to “happy”, which is more of a mood in your emotional landscapes. They depend on the whole of your person, the character traits and how you choose to manage them, the efforts and the work you do on your personality, to the discipline to regulate your behavior and parent yourself in order to build who you want to be. Happy will pass and be replaced with something else, which will also pass and give way to a new thing more or less happy. You might get pure moments of happiness here and there, but it’s a dangerous and kind of empty pursuit to actively seek for (only) happiness. If it comes take it and enjoy it but, as a fundament, work on things that are meaningful and last longer than any emotion so dependent on circumstance.