KasiCair
I mean it makes sense that the goblins aren't as powerful as witches who in turn aren't as powerful as magicians. Voldemort defeated them with ease and the magicians in Dxd are way stronger. And Wth should they care about Harry? Other then maybe defeating a common enemy there's nothing to gain from helping him but that could possibly mean risking the ire of Voldemort and his supporters. Only Flittwick is a seemingly powerful representation of his race and only due to being a half goblin.
I have a suggestion on how to properly introduce Hermione and some of her friends to the wider supernatural world. How about giving Harry the sacred gear Mirror Alice. Its nothing really powerful but can be useful I defending themselves. Plus you can use it as the explanation on how Harry survived the killing curse. That he subconsciously activated it after watching his mother get killed and when Voldemort fired the killing curse at him it got reflected by the sacred gear with double the power. You can have him meet Harry at the Burrow after being invited by the Weasleys to the World Cup and have him detect the sacred gear and tell Harry and the others about them.