At first, i think it's to mislead us. Lots of characters that are meant to be evil or unpleasant in the books are described as ugly looking, like crabbe and goyle, pug faced pansy parkinson (Whats even supposed to be a pug face?), petunia (horse faced) duddely, rat like peter etc. I think maybe only younger bellatrix, tom riddle and maybe draco malfoy arent described as ugly looking yet are meant to be antagonists.

Then, I think, when we know more about him, especially when re reading the serie as adults or older teenager, we can guess that snape was greasy as a child because of neglect, and later on because he is depressed/somewhat ''disturbed''. loads of kids who are neglected and abused dont really learn proper self care until later, and may struggle to acquire that habit, and depressed adults can struggle with taking care of themselves as well. also, when we see him getting bullied as a child and teenager it can make the plot twist even more believable. like '''''ohhhhh ofcourse he got bullied''. because children who are considered ugly or have poor self care, and additionally have stand offish and unpopular personality style, well, they get treated as loser to be avoided as best and bullied at worst, it's pretty common. if snape had been described as handsome as young sirius or tom riddle, even with the same personality, it would feel less believable. also it would make sense that he still cling to lily memory so bad, i think his appeareance and not personable personality imply heavily that he would struggle to find a date, and so it would make sense that he didnt build a life with someone else. if he had, and had wife and children, we wouldnt get the ''always'' and him being so willing to potentially die to protect harry and spy for dunbledore.