The Endbringers will hit again before she can do anything useful at this rate!
Building things to burn off frustration doesn't seem to be working this time. She asks her lawyer how illegal it would be for her to pick a fight with a villain, under various circumstances. Nonlethal force of course.
Absolutely not illegal at all, she can do that whenever, the regulations are only meant for profitable uses of her powers. Of course, if she roughens up a villain too much she might get in trouble that might be alleviated by an independent hero registration, but that one's really easy to get, just fill out one form and submit it, she doesn't even need to wait for it to be validated.
So she fills out the form and submits it, and makes a big granite-handle steel-top sledgehammer to look even more intimidating than rock armor (even heavily gemstoned rock armor) already does, and starts lurking in the skies and roofs of non-New York cities, stopping any crimes she finds and hoping to find one being done by a villain.
Excellent. What're they doing, and does their costume give any hints to their power?
Then she falls to the street outside the jewelry store with an exaggerated thump and tries to stick the villain's feet to the floor by growing rock around them.
Rock: Meet villain's gut. It's a light hit, since she doesn't know how much she needs to hold back.
One: the rock returns to the position it'd been occupying ten minutes previously.
Two: the villain apparently "flickers" in place, going back to where she'd been standing about a fifth of a second before the rock hit her and continuing to run. She stops, looks up at Mountain—who is now flying, how annoying—shrugs and returns to putting assorted pieces of jewelry inside a black bag.
If the villain gets sufficiently distracted by this, another rock hits her from behind, somewhat harder.
and she's standing exactly where she'd been before, as if the rock had never touched her. She starts jumping up and trying to boop the pieces of jewelry she can reach.
"Go home."
Needling the villain into a stalemate is almost as fun as actually getting to punch something tough, who knew?
"I don't have it out for you, you know, I'm just burning off some frustration. But just sayin', isn't there some other way to make money with time nonsense than stealing?"
She shakes her head, but it's more a frustration gesture than actually replying to Mountain's question. Just out of spite, she returns to the store and starts rewinding the jewelry she stole back to their appropriate locations. Maybe if she's fast enough she'll be able to get Mountain's powers to cause some property damage.
She loses her hold on each individual piece as it gets rewound, and simply doesn't grab them again if it'd cause damage. She does trip the villain again, snickering.
She stops flying and waits for someone Official to arrive so she can explain what exactly happened.
"Hello! I'm Mountain. I am not the one who was robbing the store, she ran away."
The shopkeeper starts explaining the part where the villain calmly walked into the store and touched the people there, making them disappear, before pointing a gun at him and lazily grabbing jewelry and putting it inside a black bag.
When he's done with that, Mountain continues, "I fused the gun then tried to immobilize her, but she just flickered around it. Did not know her power then but she ran at me so I assumed she thought it could hurt me - I flew up. Then I stopped her from taking anything by moving the jewelry around and hit her with rocks not too hard until she gave up. Those people okay?"
"Seemed like reversing time or something? She put things to where they were when she touched them. I'm not sure on the details." She draws the rewind symbol. "This was on her costume."