This isn't the first time Norse mythology has been mentioned in the show (it was previously mentioned in Hercules and the Pool Party when Zeus asked about Odin and Frigga). This is the first time Norse mythology has been seen in the show however.
When the ice monsters are shouting "Ragnarok!!!" and going to Valhalla (but accidentally heading towards Olympus instead) and Loki says "love the enthusiasm guys, but Valhalla is that way" parallels exactly what happens in Hercules (the animated movie) with Hades and the Titans