Can a kid being being unpopular at school lead to poor health?

Yes. Any situation that impacts personal achievement and confidence may lead to bad choices & bad habits (obesity, alcoholism, smoking, poor academics). Trying too hard to be popular can be the quickest way to get these habits. Try to see the good facets of yourself, & be happy with them. This will reflect outwards to others as well. If this condition is troubling you, please discuss it with ur doc.