In real life, body odor is a fact of life that is inevitable for most if not all humans. Get a little sweaty, delay a little (or a lot) since your last bath and you might accumulate a scent that at least annoys if not offend others of your kind. Otherwise, some people seem genetically disposed to a bad scent or have a disease that creates this as a symptom. However, with basic hygiene most of us do not have a scent that very noticeable in day to day life.
This trope is not about that though. Or rather, not exactly.
This trope is about humans provoking a reaction, possibly through no fault of their own, to other races like elves, dwarves or hobbits but can easily include other races, especially those with unusual sense of smell. Often among more arrogant races, like elves, this may be one of the reasons they'll cite for the inferiority of humans, or rather the superiority of their own race. Bonus points if they describe the aroma as something very unusual we wouldn't generally link to human body odor.
Due to the nature of this trope requiring the response of a non-human party, this trope is normally reserved for fantasy genres.
Note that for this trope to apply, this must be a human whose body scent must be in a state where other humans present would not object to the scent but others would. Someone who sacrifices basic hygiene or having a condition that creates a bad body odor that will offend humans is likely to provoke a justified response to anyone else having similar sense of smell (although such a odor might still further give cause for a non-human to say humans are smelly.) This trope also does not apply to artifically induced odors such as a Skunk attack, which could possibly give that odor to anything, not just humans.
Finally, the scent need not be offensive, although reactions that view the scent of human as being very favorable is likely to be rare.
I'm a little lacking in terms of examples at the moment, but I would bet there are plenty of references for those willing to look for it.
One frequent running gag in the Webcomic Order of the Stick is for the halfling Belkar (who is also a gourmet chef and thus has a honed sense of smell) to state that humans stink horribly to the point of causing him misery every day. http://www.giantitp.com/comics/oots0838.html
In this particular case, the scent he smelled that strip is unusual, but since he comments that this distress is a daily occurance the example works.
In the webcomic of Flintlocke's Guide to Azeroth, the elf Kathrena describes humans (to the fellow human Schweitzer) as all smelling as sweat and barbeque sauce and to which the dwarf Flintlocke shows up and ask if someone is cooking barbeque. http://pc.gamespy.com/flintlockes-guide-to-azeroth/episode-14-the-scarlet-hootenanny/675687p1.html