Mom wants her room to be over 80 degrees most of the time. I feel this is unhealthy. Is it?

As a person ages, and health problems become an issue circulation to the extremities diminishes. So hands and feet feel colder, arms and legs as well.
A few things you can try.
Warmer clothing. Polar fleece tops and pants. Oddly they keep you warmer without making you sweat and without the added weight of some other fabrics. And if the fabric gets wet (accident or a spill) it does not absorb the liquid as rapidly.

A hat. A knit cap or baseball cap will help the head retain warmth.

A lap blanket.

Place an electric space heater in her room. Make sure it is secure and away from any window treatments, bedding, clothing. You could put it on a timer if you want so it goes off after she is asleep and comes on as she wakes.
Many electric blankets have timers on them and will shut off after a while and do not have the electric blanket in contact with skin.
BUT do NOT use an electric heating pad if she has an neuropathy (a friends husband badly burned his legs because he could not feel the heat)

the high temp is not "dangerous" or "unhealthy" for her, she really is cold it is due to her ages, her circulation, underlying medical conditions and possibly even medication side effects.

I live in a senior apartment. We have not turned the heat on in our unit since we moved in. It still runs at 75 degrees all the time because the building is so warm. In fact, on the few occasions it falls to 73, my husband (88) complains of the cold! The younger employees are always in summer clothes.Must be why old folks in novels were always in a nook by the large kitchen fire.

She can’t help it.

Her internal thermostat is working differently due to her body function.

She is literally freezing.

Help her by finding comfortable layering clothing. Heated blankets and throws are wonderful as are similarly themed battery operated heated jackets.

Let her be comfortable.

OP has commented that the MIL refuses to wear pants. Big part of the problem. Brrr!

My mother is 98 and in hospice in AL. It is about 78 in the room and she says it is cold-- and I am sweating. It is anatomy/ biology for the age.
Lots of good suggestions here but mine is: Electric blankets are a miracle. They are very light. Get an electric lap blanket, and electric blanket for her bed (with a quilt / light blanket over it to retain heat).
She will still complain though... you are amazing for taking her into your home.

Check out thin, light weight, long underwear. My mother and MIL took to them quickly. The silky feel and lack of bulk plus ease of putting on was a selling point.

Deb4321, welcome to the forum. I remember walking into my parents house and it felt like walking into a hot flash. The thermostat was set at 80, and Mom's hands felt like ice.

Mom would be bundled up wearing thermo under garments, a long sleeve shirt, sweater, cotton scarf around her neck, long pants, knee socks, and shoes.

Dad would be wearing.... well, summer shorts, no shirt, and sandals.

Make sure your Mom has an undershirt/thermo shirt that is tucked into her slacks, tucking the shirt in makes a big difference. Same with wearing pj's, have the top tucked into the pants unless Mom is wearing an undershirt, tuck in the undershirt. Wearing thin cotton socks to bed is also a huge help. Fuzzy socks tend to make it difficult to turn over, it's like being stuck in velcro.

Add to Mom's bedroom one of those long floor heaters which are tip proof. It acts like an electric baseboard. Keep it away from the bedding and curtains. Then you can turn down the heat for the rest of the house, especially at night.

Long time ago our grandmothers use to bring to bed one of those orange hot water bottles. Some elders even wore bed hats to keep warm.

My ex-MIL has literally isolated herself from family and friends because of the heat issues. We live in southern Vermont. She keeps her thermostat at 80 and has a wood stove at the same time. None of her grandchildren or children who live in the same town visit because of the heat. I am her ex-SIL and do most everything that gets done. I hate the heat and always wear gym shorts and t-shirt when I go over there. She's a difficult woman and refuses to consider the comfort of others.

In addition to more layers of clothes, use a space heater in HER room or where she sits to keep the space around her warm and the rest of the house cooler. At 97, I don't imagine she moves much, so it's understandable that she's cold most of the time.

No, it's not unhealthy to have the temp at 80 or higher around her. In tropical areas of the world, it's common to be in 80s or 90s everyday. The bodies adjust.

I agree with a space heater, perhaps one with its own thermostat. Maybe a humidifier so she doesn't dry out like a prune.

After being "the hot one" of the two of us, my 65-yr old husband has started creeping up the temp in our home during the day in the winter. It's now at 72 degrees. He still wears multiple layers of clothes and complains about being cold. He has a space heater in his home office. He is a perfectly healthy man who still plays hockey 3 mornings a week, kite ski's on our lake on weekends, and is not skinny by any means. So, it must be a circulatiion or internal thermostat issue.

Maybe consider silk long johns for your Mom... we both have a set. Very lightweight and comfortable.