It should be pointed out that while Diane was a Tulpa for nearly the entirety of The Return, Gordon Cole, Albert, and others did not react as though Tulpa Diane was behaving out of the ordinary until the very end.

These are some pretty smart and perceptive individuals, so I think it stands to reason the 'Real' Diane acted in a similar 'Grumpy/Mean' manner. I like to think those small moments of kindness we see in S1/S2 is rare for her, and that is what makes those gestures to Cooper more meaningful. She's still not my favorite character in the series, but it wouldn't be the same without her.

I figured most fans love the character Diane because she was played by Laura Dern - a Lynch Favorite, same with Janey-E being played by Naomi Watts.