So I'm used to the fact that symbolic things like huge drops of sweat, huge aneurystic veins, etc. are more or less abstract exaggerations of intense emotions rather than literal drops of sweat or veins. But now I'm drawing a character with a big drop of sweat looking at themself in a mirror, and I'm not sure what to do. If the drop of sweat is non-diegetic and purely symbolic, it doesn't make sense that it gets reflected in the mirror, because reflection would imply that that drop of sweat is literal, physical, and exists in the world (aka, diegetic). So is there any cases where an anime/manga character having their sweat reflected in a mirror as if it were diegetic?
It would depend on the medium and tropes at play within the particular series. Such as comedies breaking a 4th wall.
Generally. I would say no. The reading or viewer of a particular series is an extra-diegetic spectator. The narrative present are convey some sort of atmosphere to the spectator (perhaps for amusement). The emotive transformation tropes is intended as a means of distanciation, as the apparent "reality" of the on-screen world is undermined by such an occurrence.
While this is a performative metamorphosis, regardless of what is reflected upon a the scene or upon an object, the character retains their so-called "diegetic integrity", since these emotive transformations are not overtly acknowledged within the story's world, as other on-screen characters do not generally react to the emotive transformation of a character (e.g., what's up with that sweat drop; why do you have that angry knot on your head if you're not angry).
They are aimed at the extra-diegetic spectator audience. Much like how a third person narrative is understood to be outside the a diegetic action, at least in regards whether the narrator and audience are aware of of the character, but the characters are not aware of them. Much like how in a work of literature the narrative voice is bound to the presentation of its characters.
In short, while the characters remain as they are, oblivious to any scrutiny from the audience, their existence is tied to (inextricably) to communicating with extra-diegetic spectator of an audience. These emotive transformations serves as emphasis to an external emphasis of internal expression.