Ha, was sort of punking you this time since Diane gets a ton of shit for being underdeveloped, but i am also somewhat serious now that i think about it. Diane is genuinely a great character, better than Meliodas, and better than Elizabeth.
Putting aside her general perky and enjoyable personality that i do like a lot, my main reason for liking her is similar to the reason i like Elizabeth, an extraordinarily talented member of her race but also an outcast of said race, she doesnt like killing for no reason but grew up in a society centered around death and violence. But where she surpasses Elizabeth imo, is the fact that she got actual development towards this aspect of her. For Elizabeth we are just told "oh she hates killing and decides to talk to demons instead", but with Diane we actually got an on the ground experience of Diane on the battlefield told to fight and kill, we actually got to see her struggle with it. And even better we got her friend Dolores who was even more adverse to killing than she was, get killed as a result her aversion to battle, and Matrona, the woman telling her to kill, to fight and not worry about the lives of her allies, lose her life for Dianes sake. To me that whole aspect of Diane really is just a more reasonable and fleshed out version of Goddess Elizabeth's ideals.
Oh and i loved kid Diane's whole backstory with King, and how it was eventually tied together with her crush on Meliodas and how she was given her sin of envy, was really a great moment when i read that. Her whole thing with Gowther and Dolor was alright as well, running away from Zeldris was a good way to end her trial.
So yeah, while Diane hasnt had a great deal of development and i wish Nakaba would spend more time on her, the development she has had has been great imo, and quality>quantity for me.