The fossorial people are small humanoids. They have hair on their head, but it only grows to around shoulder length. They also have similar hair on their chest, hands, and feet. This hair can resist dirt. Their skeletal characteristics are described here. What clothing would be best at resisting dirt while crawling through or digging dirt-tunnels, while still keeping the wearer warm in an environment akin to medieval Britain?
Why Wear Clothing at All?
Consider normal nudity. If these people evolved to live underground, then they've probably grown accustomed to dirt, insects and the like. Clothing is a matter of culture more than anything else, and seeing as humanoids (bigger ones) have historically gotten along just fine & naked (Tierra del Fuego), there's no obvious reason why these people must wear clothing.
Apart from difficulty in obtaining materials, in the darkness, clothing won't be useful for signalling or decoration (since it's so dark) and anyway, everyone will just misplace their socks.
It's easier to go without!
The only fibers, or fiber like structures, which they can access underground are roots.
The roots of adequate diameter can be dried and woven together to produce a very rough fabric, at most resembling yuta bags, which can offer some protection against contact with the underground. This is what they probably actually need, since a few meters underground temperature is pretty much constant all year long with very small fluctuations when compared to the changes above ground.