I think you can be poor and happy and also rich and miserable. And vice versa. I think it's all relative, tbh.
You could call me poor going on my earnings. I really struggle some weeks/months, but I'm not at all unhappy. I think your approach and mindset play a huge part in how you deal with certain stresses.
I could quite easily curl up under my duvet and sing a woe is me song about all the things I don't have and constantly wobble over my dry bank account, but what's the point? Is it going to solve anything or make me feel any better? No. I know it sounds horribly cliche, but you really do just have to make the best of things sometimes. Problems don't fix themselves.
I can't afford a great many things. I'm usually always behind with one bill or another and when it's the final few days before payday, I am usually hunched over the kitchen table counting out coppers from my penny tin to buy bread and milk. The only reason I haven't been evicted for stumbling with my rent most months is because my parents are my landlords and they're fortunately very understanding. I wish i could say the measly £5 I receive every week in child maintenance goes straight into my daughter's savings account, but unfortunately I always need it to buy food, etc. Hey ho.
I do have my moments now and again where I get fed up with the struggle, and then I remind myself that things could be so much worse and give myself a good shake.
I think part of being a flawed human is our desire to always want 'more'. It doesn't matter how good we've got it, we constantly want bigger and better. And living in that mindset is what makes people so miserable, because nothing with ever be enough. There's a lot of peace to be found in acceptance, I've found.
Are money and happiness linked? Yes and no. I say this because I don't believe material goods and an obscene fortune ever buy anybody genuine happiness, but day-to-day stability in life always will.