Often in anime which features a character who wears glasses, when they take off their glasses, their eyes turn into a sort of puckered lip emoticon that looks like the number 3.
The meaning is fairly obvious, that the character looks drastically different or worse without their glasses on. But why is this icon used, specifically?
The "3" is a stylized (and humorous) way of showing the characters squinting their eyes, which then look somewhat like a "3".
Most glasses wearer — especially the young ones — are short-sighted, so whenever they take off their glasses, they start squinting. The "3" is used in a similar fashion to depict pouting characters, like with the following emoji:
(°3°)
A variation of this is the famous へのへのもへじ (henohenomoheji), which uses Hiragana characters that look somewhat like facial features and is used almost exclusively for humorous purposes:
Image taken from RocketNews24
In this scene, Marii goes on to ask "Why the Nobita Face?". The fact that she refers to the trope as thus would indicate that it's the origin or at least the most famous version of the trope.
From a previous question I asked:
From the Doraemon Wikia:
Nobita's eyes are never drawn consistently. Sometimes they look like the number 3, sometimes they're black beaded, and sometimes, like in the case of the recent movies, they're normal eyes.
