How does one become worthy of Valhalla? By dying a brave death in battle against impossible odds. When Dewey refuses to back down from Jormungandr at the end of the episode, he proves that he has the will and the courage to do exactly that. No wonder he won over the crowd at the end.
By extension, this could also explain why the crowd initially took an instant dislike to him; you don't get into Valhalla with charisma, catchphrases, or other shallow displays. Of course they would look down on a showboating braggart who expected praise and respect before he even lifted a finger.
Why aren't the Ducktales version of the Norse gods fighting Jormungandr, particularly Thor as he’s its nemesis during Ragnarok? Team Earth forces only champions from Midgard aka Earth to face Jormungandr in exchange for facing its more vulnerable form of a wrestler instead of its gigantic world crushing form. There's likely a Team Asgard to handle its bigger form.
If Scrooge is worried about infighting between Donald and Della, why didn't he bring just one of them while the other is kept busy? The sight of the kids getting hurt would have triggered the rage in either one of them, likely resulting in a Curb-Stomp Battle that might have gotten Team Earth disqualified for overdoing it.
Alternately: Scrooge was planning for this trip to be a learning experience, the kids actually having to fight wasn't part of the plan. He left Donald and Della at home because they can already handle themselves while the kids still have some growing to do. He lied about Donald and Della to make sure the kids took this seriously.
For all we know, "The Rumble For Ragnarok!" is the only time Jormungandr leaves his giant snake form. We never see him move in said snake form. Let's hope he is unconscious in that form...
