Some people are not always what they appear. Some people do things so they can stay off the radar of the man. Just because you are a college kid and like to go hang out at a coffee shop does not make a person a yuppie. Some people maybe selling drugs, just saying.
No one can really say what a yuppy is or is not. People can identify a few things associated with the image and apply the label, but it is just too large of a group to really describe a type of person. I think the term has become something people use to describe anyone who annoys them.
No, the term "yuppies," is not too general, although it would include individuals working in many different sectors. The term "yuppies" refers to young, urban, professionals, which do all somewhat fall into the same class, regardless of the sector in which they work. So for that reason, yuppies does specifically define a group, as opposed to be too general in referring to a group.
No, I do not think yuppies as a descriptive term is too general, because we all know what it means. A yuppie describes a somewhat wealthy person who is up on the latest trends. We know that they are wealthy and that they believe that they are hip and cool. It is not too general of a term because it accurately displays the meaning.
Yuppies as a descriptive term is quite specific. People imagine white Americans who are white collar professionals, dress in business wear, belong to country clubs, and have passions for the arts as yuppies. It is not derogatory or a generalization. There are a lot of people who fit this category.