I’m currently leveling a toon through Stormheim and got to the Helheim section. Once I got the Kvaldir Disguise I no longer can mount at all in Helheim. I have waited for this buff to wear off (since you can’t dismiss it) to no avail. I see other people mounted up in this zone so I don’t know why I’m glitched. It says mounts are not allowed here in my mount page.
It might have more to do with the disguise than the zone.
Later, you can return (I think for more quest chains, and certainly for World Quests in the zone), and mounting isn’t a problem then. There’s no disguise at that point.