Meaning and purpose of the flying saucer in Fargo season 2

Deus Ex Machina

The Fargo universe is indeed full of Deux Ex Machina (unexplained luck) occurrences. For example Lorne Malvo disappearing from Lester's basement or Stavros finding the money in the first season, or in the third season :

Emmit's car reworking again.

So the UFO on this scene is probably another occurrence that allow Ed and his wife to escape.

UFOs on Fargo

Now about UFOs, you can remember this is not the first time that UFOs are mentionned on Fargo. I won't spoil, but you'll find a lot of references since it was a common trend at the time of the second season is set.

Nah Hawley explaination

About this perticular scene, Noah Hawley answered:

“What was the deal with the UFOs?”
At first, Hawley tried to brush off the question. “What was the deal with the UFOs? What was the deal with the fish falling from the sky in the first year? I mean, these things happen.”
“Yeah, but you explained [the fish],” Willimon said. “Granted, it was a tornado that hit a lake, but there was an explanation.”
“Well, it was part of the moment,” Hawley continued. “Post-Vietnam, it was that both the political paranoia and the conspiracy theories went all the way to the top — with Watergate; that sense that people were feeling paranoid on some level.”“If you look at the internet research device, there was a state trooper/UFO incident in Minnesota in the ’70s, which I thought was interesting,” Hawley added. “And then also Joel and Ethan [Coen] had included some of those visuals in ‘The Man Who Wasn’t There.’ There was definitely a UFO runner in there.”
But the truly interesting take came straight from the Coens’ original film.
“Very early on, I asked, ‘What is our Mike Yanagita?'” Hawley said. “Mike Yanagita was the character in the movie ‘Fargo’ who Marge met after being friends in high school and they had a meal, and he talked about marrying his high school sweetheart and then she died and he was so lonely. But then, later, you found out he made all that up. And I thought, ‘Why is this in the movie?’ It has nothing to do with the movie — except the movie says, ‘This is a true story.’ They put it in there because it ‘happened.’ Otherwise you wouldn’t put it in there. The world of ‘Fargo’ needs those elements; those random, odd, truth-is-stranger-than-fiction elements.”
“Especially because the storytelling is so compact, so tight, that it can’t be too tidy,” Willimon chimed in. “You have to add some untidiness to it.”
“Whenever you introduce those elements, you engage the audience’s imagination,” Hawley said. “When you’re not spoon-feeding a linear story, when you’re leaving gaps for the imagination, the audience is going to have to invest more in it. And I think that dynamic relationship is much better than just watching.”

Is this some sort of Deus Ex Machina?

Essentially, yes

Noah Hawley, the showrunner for Fargo, explained to Entertainment Weekly

The Coen Bros. sometimes put something in because it’s funny, but that doesn’t mean it’s meant to be comic. … There’s a couple things that felt right about it. One is that it plays very well into the conspiracy-minded 1979 era where it’s post-Watergate, you had Close Encounters and Star Wars. There was a Minnesota UFO encounter [in 1979] involving a state trooper. It was certainly in the air at the time. Alternately in the Coens’ The Man Who Wasn’t There they had a [running UFO thread]; certainly it was more ’50s inspired, but it was part of the cinematic language of their movie. So it felt like it worked for the time period and worked for the filmmakers, and is a way of saying “accept the mystery” — which is a staple of the Coen Bros. philosophy in their films. And I thought it was funny. But obviously it affects the story in a very real way. It’s not just a background element.

[...]

At the end of the day, Peggy’s line sums it up — “It’s just a flying saucer Ed, we need to go.” I like your “I don’t know, I need to think about it” reaction. So much storytelling, especially on television, is a spoon-fed experience with clarity of all things. You’re going to have to see the end of the story and look back at it and ask how you feel about the deus ex machina of a UFO saving Lou Sovlerson’s life and what would happen if it hadn’t. I think those elements in a story are really exciting because we’re so unused to having them. We usually separate our genres more neatly. To suddenly have a genre element come into a dramatic story is exciting.

Also at IndieWire

“Very early on, I asked, ‘What is our Mike Yanagita?'” Hawley said. “Mike Yanagita was the character in the movie ‘Fargo’ who Marge met after being friends in high school and they had a meal, and he talked about marrying his high school sweetheart and then she died and he was so lonely. But then, later, you found out he made all that up. And I thought, ‘Why is this in the movie?’ It has nothing to do with the movie — except the movie says, ‘This is a true story.’ They put it in there because it ‘happened.’ Otherwise you wouldn’t put it in there. The world of ‘Fargo’ needs those elements; those random, odd, truth-is-stranger-than-fiction elements.”