Is a witch hat a proper noun?

No, a witch hat is a common noun, a Word for any witch's hat of any kind, anywhere.

A proper noun is the name of a specific person, place, thing, or title; for example:

• Witch's Hat Backpackers Hostel, Perth, WA Australia
• Witch's Hat Brewing Company (bar), South Lyon, MI
• "The Witch's Hat" by Tony Johnston and Margot Tomes