Call somebody a bitch, and they might or might not actually be one. It's a matter of opinion, up for debate.
But smelly is smelly; the vast majority of people agree that certain odors are unpleasant or disgusting, and aside from very little kids who might just say things like smelly or stupid or fat or ugly because they are general insults and not necessarily true, calling somebody smelly means exactly that, that they smell bad.
And being called smelly is especially insulting because it calls into question that person's hygiene and possibly their good sense. Like implying that they haven't got the sense to wash regularly or use deodorant, or that their home is unclean as well.
Remember "Harriet the Spy?" After the other kids got hold of her notebook and read all the nasty-but-true things she'd written about them, they started picking on her, and one of the things they did was pass a note (which Harriet intercepted, probably as they thought she would) that said she smelled. This bothered her so much that she actually got excused to go to the bathroom, where she sniffed herself all over and even washed her feet.
People do not want to be smelly, never mind get called smelly.
You're entitled to your own opinions. You're not entitled to your own facts. -- Daniel Patrick Moynihan