It seems like some people are able to see through the glamour, but it is not common.
The difference between the Innate Glamour and the "HEIMERDINGER’S VISUAL TRANSMOGRIFICATION" (as it says in the Universe page), its that "Yordles do not necessarily control their glamour, and may understand it in a variety of ways. The revered inventor Heimerdinger, for example, crafted unique technological devices for himself and Ziggs, to help them blend into Piltovan society."
In the case of Poppy, the story "The Slayer" explains how Demacians percieve her... even if Poppy is unaware of it.
That story implies that the Slayer that the towns venerates, is Poppy, something that is pointed out by a little girl.
“Look, Da. It’s the slayer! The one from the statue!” declared the wide-eyed girl.
Poppy saw the girl was pointing in her direction. She whirled around to see if the slayer was standing behind her. But no one was there.
Hower his father tells the girl that:
“No, lass,” said the girl’s father. “That one’s no monster slayer. Too small by half.”
The girl and her father quickly lost interest and strolled through the village to partake in the various amusements.
The father doesnt say that she cant be the slayer because she is a yordle, he says that it cant be the slayer because Poppy is too small... So, he is admiting that Poppy looks like the statue of the Slayer, but a smaller version of him.
So, basically Poppy is using her glamour, even if she is unaware of it, and she looks like "the paragon of a (small) Demacian warrior—armored in heavy plate mail, and rippling with sharply defined muscles."