I'm with you on this, and then again I'm not. The '55 Chevy was a beautiful , and clean design that far outshines the '56-'58 versions in my opinion. However , I don't see linear decline in design with each year a touch worse than the one before. I think you have to take each year as an individual case. The '56 was just a tarted-up version of the '55, and not an improvement. But the '57 really works as a design - there is a reason that it has been an icon for so long. The '58 always looked fat and overblown to me, but I've warmed to its design over the years, especially in the lower trims.
Then there is the '59.
This is the game changing year, not 1960. It's radical and brash with huge fins, and lots of chrome, but under the gingerbread it was a revolution in design. The stance, the dimensions, the underlying shapes - all of it was wholly new. Yeah it's a bit polarizing, but WOW!. I find it to be a milestone that easily equals the '55 (though in an entirely different way).
As far as the late 50s in general go, I'm with you. Many of those cars have always looked like overblown tubs slathered in chrome that didn't compliment their basic designs. But there were plenty of exceptions to that rule. 1959 changed all of that, but then Studebaker had already moved in that direction in '53, or thereabouts. And the late '50s Chryslers had already tidied-up their act. C1 Corvettes never really suffered the general design bloat of the 50s at all... And even on the worst of the fins and chrome crowd, there were thoughtful and interesting design details. Lower, longer, and wider ushered in my favorite era of design without any doubt, but there are earlier gems.