Maxperson
Morkus from Orkus
An element, yes. I would not say core. I have the 2e boxed set and very little is dedicated to the mists going to other worlds. It's really just a minor mention. The core elements of the mist are separating the islands from the main group of domains and it being really hard to travel the mists without Vistani.A core element of Ravenloft is that the mists reach into other worlds to pull in great evils and hapless adventurers, who can seek to escape. You can ignore that part, but it's pretty deep in there.
The domains are all said to come from elsewhere, but you don't have to do that. You can have a 100% Ravenloft game without ever getting into travel to or from other worlds. You don't even have to go out of your way not to mention it. Just run a party born in Ravenloft through Ravenloft domains. Other worlds simply don't come up if you do that.