Hello
You're right "gueule" refers to the mouth of an animal but in informal French, it can also refer to the mouth or the face of a human being (more often the face when it's a human). "a mouth of joy/fright" doesn't have sense in french but "a joyful/frightened face" does.
So "une gueule de joie" is "a joyful face" (or "a face of joy") and "une gueule d'effroi" is "a frightened face" (or "a face of fright")
But "face" is more formal than "gueule" so if in English there's an informal word for "face" that would be a great translation for "gueule" in this song.
Moreover "blanc" in french is yet the color white but also refers to the white-out. As she was singing about chalk and cheat sheet I think there is a school theme in this verse and that "blanc" is the white-out
I hope I helped you to understand a bit more this song
*Apollinaire is a French Poet well known for his poems written in calligrams - an illustration of an object made out of words related to said object. (Search it up, it’ll make a lot more sense)
Also, “Une gueule” is a mouth of a large animal, usually reptiles. In the song, it’s used to emphasise the large smiles and large “frowns” as the singer goes through ups and downs of life.
To clarify a lexical ambiguity of “A cocktail of fear which fades”: the thing that fades is the fear. So, in a less elegant and more accurate translation: “A cocktail of fading fear”.