Personally, I think the upper body of paper bag pants looks good with any kind of T-shirt, which can be paired with a fat T-shirt or a thin T-shirt, depending on the individual's figure. It's better for fatter people to wear a bigger T-shirt, because the pants in the lower body feel bloated, so it's better to wear a wide T-shirt.
I once saw a model wearing a T-shirt with princess sleeves and felt very stylish. If you are a thin girl, you can try this feeling to make your figure more rounded. If it were me, I would wear a loose T-shirt, because the upper body is not thin, and a loose T-shirt can better decorate the lines of the upper body. Moreover, it is more convenient to exercise. It is more appropriate to do some housework at work at ordinary times.
If you are a younger girl, you can wear a colorful T-shirt on paper bag pants, because this collocation will make younger girls more energetic and youthful.
If you still don't know how to match, you can refer to fashion magazines or online fashion websites to get good inspiration. You can also get a matching plan through entertainment magazines. Because every year, when stars come out with new products, professionals help them design clothes. I remember that Li Bingbing wore a pair of paper trousers, but not a T-shirt, but a white casual vest. Personally, I think her collocation is also very fashionable.
For girls with good figure, you can try this method, which is very youthful and energetic. It is also suitable for those beautiful young girls.