dthunt is right. The prominence a female character has in the storyline before her death doesn't really affect whether it is or isn't a fridging. What matters is whether her death is a narratively appropriate end to her story and character arc, or whether her death primarily serves to motivate a male hero and further his heroic journey. (See http://www.feministfrequency.com/2011/04/tropes-vs-women-2-women-in-refrigerators/ for a full discussion.)

Hermione's death was a textbook fridging. That's not to say that it's automatically offensive or sexist or whatever: like TVTropes says, tropes are not bad, tropes are tools. But yes, Hermione's death is a classic fridge death.