Average happiness would still be happiness. Unhappy would be not happy, but not necessarily sad either. There’s lots of different ways to be unhappy other than being sad. Unfair just means something isn’t fair. But something being fair is often treated as a binary. Tall and short isn’t a binary, so simply not being tall doesn’t mean something is short. But in the context of a joke like this, I’d assume “not tall” would be used to be as a comedic understatement.
Average happiness would still be happiness. Unhappy would be not happy, but not necessarily sad either. There’s lots of different ways to be unhappy other than being sad. Unfair just means something isn’t fair. But something being fair is often treated as a binary. Tall and short isn’t a binary, so simply not being tall doesn’t mean something is short. But in the context of a joke like this, I’d assume “not tall” would be used to be as a comedic understatement.