Pinkie Pie apparently regularly eats special 'Rock Candy' which has a secret ingredient: a 'Special Kind of Rock' that Maud 'Discovered'. This is the Crystal Meth Empire all over again, isn't it.
On the first watchthrough it looked like Maud was just trolling the ponies, but it eventually occurred to me that a lot of the stuff she does is stuff that Pinkie Pie would appreciate. "It's like hide and go seek, but way more intense!" No... it's a way to entertain Pinkie Pie while getting her to bring you every rock in the vicinity. When the other ponies started complaining, she ended the game by finding Boulder. She probably didn't notice that they were getting bored and frustrated before that because she's probably not good at reading emotions more subtle than Pinkie Pie's.
...okay, she can probably read Rainbow Dash.
The other place where it looked like she was being deliberately cruel (with the apple peeling) was probably the same way.
... the dishrag did go very well with her complexion.
"He's a rock. His name is boulder."
"This next one is about rocks. They're all about rocks."
"I think this one is done. Should I peel another?"
The part where she nudges Boulder at the sandwich as if he can eat it.
And obviously, Maud uses Rock Smash. It's super effective!
That said... there were parts of the episode that really didn't work. Where the pacing was and dialog just awful. "She's like a secret wrapped in a mystery wrapped in an igneous." Ha ha ha! Good joke! Now stop right there. No... no, Rarity, stop. Rainbow, stop, you're over-explaining... okay, it's dead. You killed the joke dead. Why are you still going?
That happened more than once, and by their very nature they seemed to go on forever each time. x.x
So I guess it's 2/3 overall. The bad parts really did kind of ruin it for me, but not all the way.