I just bought this pilea today at walmart (i know i know)

it looked even sadder when i bought it but i want to try to bring it back to health. i cut off the sad looking leaves and left the ones that looked semi-healthy.

can someone help me figure out why the leaves look splotchy? i was thinking it was overwatered but the wilting makes me think that it has been underwatered.

You'll never know what it suffered, but what's important is now. Keep in the same pot, don't overwater, give it enough light. I'm not familiar with this plant, someone else will help with that.
Any retail store can have neglected plants. No one here (should) criticize you buying from Walmart. Even the big boxes with a million plants will have failures. Heck, there is a whole ongoing thread of people crowing over their purchases from the clearance rack of sick, overflow, and dying plants

Does that pot have a drainage hole?

Take it back.

LOL, I agree, it doesn't matter where you purchased your Chinese Money Plant (Pilea peperomioides) I've purchased plants from Walmart, Home Depot and Lowes, as well as nurseries over the past 50+ years and I've noticed for many years that the big box stores often don't properly care for their plants. I've seen plants so very dry that they are crispy and almost dead and I've also seen some very sad plants with soil so extremely soggy from being over watered. There are many of us who can't resist a "rescue" plant that catches our eye and we want to try to bring it back to life; sometimes it works ... sometimes not.

That being said, there is hope for your Pilea peperomioides. It needs excellent drainage to thrive and if it's in an enclosed container with no drainage holes, the roots may very well be suffocating. The drooping and splotchy leaves shown in your photo make me think that it's definitely an issue with water and improper drainage.

If the pot doesn't have any drainage holes, I'd remove it keeping as much of the roots and soil intact as possible and place it in a small plastic nursery pot that has
drainage holes in the bottom. Place it in a brightly lit location and do not water again until the top couple of inches of soil feel dry to touch.

I repotted it when i got it, and it does have good drainage now! i think it'll just see how it'll do in its new home for right now, it probably has some adjusting to do!

What did you do to give it better drainage?

it has a good amount of lava rocks at the bottom of the pot.

Not the same as a drainage hole !

House plants need holes in their pots. Some people manage to get away with growing in closed containers somehow, but for most, even the greenest of thumbs; pots without drainage holes quickly kill plants or even worse, they do it slowly.

I would suggest use a container with drain holes. Do not block the drain holes with rocks, since it will just increase perched water table. Add some pumice to make the media grittier and it will allow better airflow at root zone, especially if you are using a ceramic container that tends to hold moisture much longer than needed. It does not like to be in a too moist media, so learn to water with care, allow watering intervals.

As to lighting needs, a bright light area, near to the window, but not too much direct sun.