I mean, you can. You can judge anything by its cover; a book, a person, a movie. However, it's just not something you should do. Nobody's telling you not to, they're just telling you that it's not right and proper to do it.
Hope this helps!
Have a good rest of your evening! :)
You should NEVER prejudge/Judge the worth or/ value of something by its outward appearance alone.You never know...what you might find/see !!!
