And the ending sucked.
Also, what's the problem?

I didn't mind the beginning, but the ending showed a complete lack of inspiration. I guess they had to end the film somehow, and well they did. 7/10 stars for a film that really had me going before it dropped me with a thud.

Then she could have been in the car going to buy groceries and we would have had even less backstory.
I don't have any problem with the bottle being there (other than the fact that she probably couldn't actually make a Molotov that would explode to that effect), nor do I necessarily think something like that is lazy writing. Now, it would have been if that one bottle was the only thing that would destroy the alien ship, but there wasn't anything special about it other than the fact that it was in the truck (which Howard even early on tells Michelle that it's in there).

A boyfriend we never see, which is just an excuse to put her in a car, and to have in that car a bottle of booze that she can later use to blow up and alien spacecraft.
If you don't see the start and end as lazy, well that's on you.

A woman running away from her fiance (or boyfriend?) which later leads into her conversation about how she typically runs when encountering something difficult without actually doing anything about it. Then she gets hit by the truck. I'm not sure exactly what's lazy here.

Felt like it was tacked on (which it was), and the ending even more so.
There's efficiency, and that's often a good thing. But then there's expediency, and such economy of story telling that it is apparent it's a lazy setup, or in this case framing for what followed.
It was very lazy, and so was the ending, but you get that when you take a script and just rewrite the start and end to lazily fit your aliens franchise narrative.

What's wrong with the opening then?