Truthfully, between the brothers, Dean is actually the one with any ounce of sewing skills. Mallory always took care of it for her tall boys, and Krissy helped get Sam a few new sets of clothing when he was last at Trails West. 

On the other hand, Dean dealt with being a hunter on his own for years before he met back up with Sam. Clothing rarely comes with the number of pockets he uses, and he has to repair his clothing a lot from being on the job. Bobby was sure to make sure a young Dean knew his way about a needle, both with clothing and skin, while he was growing up, and no matter how much Dean bitched about it.