In my experience, I have always met a person who likes even the most disgusting smell. Time passed, and I met a person who loves an even more disgusting smell. But in more general terms, I think most people, through education, have absorbed the concepts of what is pleasant and what is not, as well as what is good and what is bad.
Well asked question. There are odors that are objectively unpleasant for the body. They are usually associated with a variety of harmful and poisonous substances. And they became unpleasant as a result of millennial evolution.
But there are no smells objectively unpleasant for human consciousness, these are all conditioned and unconditioned reflexes.
There are. For example, thiols, which are contained in the smelly secretion of a skunk, which are added to natural gas (which, by the way, does not smell at all initially; this is to feel a leak), the smell of ammonia, some carboxylic acids (acetic, valeric) and other organics.