Generalising here but in the school I work in the most popular children are generally several or all of the following:
Sporty, Clever, Pretty/Handsome, Musical/Dramatic, Confident, Friendly, Outgoing, Bubbly and Energetic.
The less popular children are often several or all of the following:
Shy, Lacking in hobbies, Quiet, Dull, Apathetic, Nervous, Tale Tellers, Whiny, Aspergers, Hugely physically different in some way, Liars, Untrustworthy.
Surprisingly, although good looks seem to help a child be popular, weight and appearance problems that can cause bullying don't seem to affect general popularity. Not at this age (7-13) anyway.
I teach in a private school btw. Perhaps, in some schools, being clever would get you on the unpopular list instead of the popular, I don't know. We are also a smallish school (about 350 children) so, although there are children who are very popular, there are none who have no friends and are completely unpopular.