If you're in the same room the whole time and check them regularly it should be fine. Cats love to be comfortable and they will move if they get too hot.
We got one of these heated mats from the pet store and our cat seems to prefer it over the space heater. It activates and warms up like an electric blanket when it senses the weight of the cat on it.
It should be fine my cats do it all the time they like warming up by the heater like people do. They have sensory hairs to know the temp.
That being said one time my cat layed directly on a heating pad on high heat for a little too long and burnt some of the hair off their skin. It grew back but yeah that's the one time a cats ever had an issue from heat in my life.
As the old saying goes, if you are cold go find the cat. She's already found the warmest spot in the house. My childhood cats used to fight over a hammock we had hanging over one of the radiators, it never ceased to amaze me how long they'd spend in that thing and the heat never bothered them. If you are concerned kitty is going to burn herself, invest in a fire guard which is a mesh barrier usually used to stop toddlers getting to the fire but is also effective for curious cats.
They almost remind me of those "greedy" slime things and also the potato thing where you put a potato in a dark labyrinth with only one correct path and it inevitably finds it and its tuber extend until its outside the box again.
Sort of like that Joan Rivers joke about having sex with her partner while they're in seperate rooms doing their own side activities.
Edit: also, vertical mazes with water being poured in
My cats both like to be near the space heater when its on. So far we haven’t had any trouble with it, but we always stay in the same room and keep an eye on them.