It's interesting, but it seems that cars gradually change between boxy and bubbly and back again. Always. There's never a fair mixture of the two styles. One always dominates, and in transitional periods it's hard to tell whether the angles are hard or soft.
Look at some of the first cars. All very, very rough on the edges.
This continued for a while until they started to round off, still keeping an overall boxy shape in dominant areas...
And then they went full bubble.
But boxiness eased its way in....
And the pattern continued as they became boxy. Some intervals took longer than others, like this boxy phase, but still.
Really, I don't think they went back to being bubbly again until the '90s.
As abruptly as they faded in, the bubbly cars began to go away again.
Now everything is angry and rigid and sharp again.
I'm sure you all could come up with a billion counterpoints, but I do think there's a trend here.