It is, in fact, possible to make cloth out of nettles. Nettles can be harvested for their fiber, just like flax. In many ways, they are superior to flax. Nettles can grow in places that cannot grow cotton, and were once widely used for textiles.
Nettle was commonly used historically, though use declined with the rise of cotton. In fact, the German military actually used nettle for their uniforms in WWI due to a shortage of cotton.
Unlike cotton the seeds are not the part of the plant that provides the fiber. Instead, the fiber is provided by the long stem (the stinging leaves are not used). Like flax, removal of extra bits of step is initially done by allowing the unwanted portions to rot, and then removing an excess. Unlike flax, nettle is a perennial and does not need to be reseeded each year. The thread spun from nettle can be used alone, or in combination with other materials.
Nettle fiber forms a stronger cloth, which unlike linen increases in strength when wet. Additionally, the fiber contains a hollow interior which makes it a superior insulating fabric, better for staying warm but still breathable. Finely spun variants were sometimes called Nordic Silk.