I don't think straight horizons are overrated; if anything tilt seems to be getting overused. I think it's a basic skill in using a camera to hold it straight and frame shots to make sure everything's straight that should be. Like a lot of effects, tilt's better used sparingly and for a specific reason.
Looking closer I did see a break in the horizon line so it might not actually be perfectly straight, but I think a long horizontal line across a photo is noticeable if it's not straight. Technical aspects if not done well can distract from the composition and artistic aspects.
The perspective overall is unusual, it did make for an interesting photo. What I disliked more was the grayness to it, I read it was shot in infrared but it didn't look like it to me. Although reading the description helped make sense of how it was shot.
I'm not sure that the title Migrant Mother was meant to mean that the woman photographed was pictured with her children, but more that she represented mothers at migrant workers' camps. There was recently a special on PBS about Dorothea Lange; she had been out taking pictures that day, passed that camp, and turned around and went back. She interacted with the people and I think that's how she got some of the photos she did.
.