I guess most people just want to read the comic, and I don’t like putting too much exposition into it, so I’ll explain it here.
This is why Mistel’s and Rick’s worlds are not in the same point in time relative to each other in the universe. To make it easier, I’ll use rough timeframes. These are not canon, just for explanation.
The Hulders’ homeworld, where Rick is now, is the “ground zero” of this time arrow. Let’s say this is about 2 million years ago from Earth’s perspective.
At first, Lodur only searched through space. He found worlds that had the right conditions, but were in chaotic phases, like ice ages, heat ages, or toxic eras.
Then he discovered that the portals could move through time as well as space. So if he arrived at a world during an ice age, he could simply move forward in time until conditions became ideal.
He could not go back in time, as that would create paradoxes. Only forward. This is why he encountered worlds where he saw civilizations rise and fall.
Ideally, the Hulders wanted empty worlds, but sometimes they arrived during a world’s civilized era.
Another way Lodur avoided time paradoxes was by ensuring that no one could reach these worlds through normal travel before the portals did. For example, if a world is 10,000 light years away, he would place the portal at a point in time that could not be reached by conventional means before its fixed arrival.
Lodur lost control of the portals when Niflheim was destroyed. Niflheim functioned as a kind of router between the connected worlds.
Because of this, even if Mistel’s world is 10,000 years apart from Earth (not light years, but time), Niflheim makes them relative to each other through the portals.
If a portal collapses, those worlds are effectively lost in time.
So Mistel’s world might be 10,000 years in Rick’s past. If he could somehow travel there directly from Earth, she would already have been dead for 10,000 years.
Inspired in part by ideas like Dr. Ronald Mallett’s work and films like Primer—this is basically a galaxy-sized time machine.