At this, Winky howled even harder, her squashed-tomato of a nose dribbling all down her front, though she made no effort to stem the flow.
I think "her squashed-tomato of a nose" might refer to Winky's snot, that is, her snot is something like "squashed-tomato". Is my understanding correct?
From Harry Potter – the Goblet of Fire — Chapter 21
Best Answer
The meaning is
Thus
"His/her something of a something" is a phrase commonly used to compare a face feature with some object. Examples from Google Books found using "his * of a nose/eye/ear":