With the details of the main character’s appearance, looks, personality, family, etc. done and written well enough to convey what you, the author, wishes, is it important to describe the other characters the same? Should you? Should you not? Is it really important? Does it matter? The answer: it is all up to you.
Now, some of your readers may skim over that description of a less important character. Then, there are some of us readers who like to know exactly what everyone looks like in the story. This makes it easy for us to picture the scene in our mind’s eye a little better. However, there are those of us who like to let our imagination run free in a story. Whether it be what area, place, building, etc., the characters may be at, or just simply what we believe the characters would look like. Then again, it really depends on the writer. It is all up to him and/or her.
Here is what I believe: don’t just describe the main character, unless the character is unimportant and does not have a huge part in the story. If you do not agree with that, at least give the less important characters a tiny bit of description. Again, this helps different sorts of readers. This helps readers get their imagination running, or others a little bit of what they want to help them make the picture. Very unimportant characters, however, do not need a description.
This is what I try to goal when storywriting: describe the main character, get the audience to have something in common with said main character, etc. However, if there is a crush my character likes, I would describe the crush’s looks. If there is a bully because he/she looks absolutely stunning and picks on the less ugly, I would add some of her/his looks when talking about him/her. I do not have to go full out, telling their measurements, etc. Give the protagonist(s) the main and biggest spotlight, and don’t be afraid to give a little to other characters.
What I am really trying to get out with this is the following: describe what YOUR character looks like, and if you picture any other characters that have to do with the main character’s story specifically, add that. That is pretty much it. It is all up to you in the end. I just advise you, if you picture a scene, character, etc. in extreme detail, add that into the story. It is your imagination, your book, your story, your characters, etc. Make it yours.