That said, there are cases where automobiles with designs that are considered outstanding by consensus proved to sell poorly. Examples include the 1936-37 Cord and 1953 Studebaker Starlight Coupe. But even here, marketplace weakness can be explained by factors not related to styling (radical engineering and an underfunded company for Cord, and quality defects and sales pressure from GM and Ford for Studebaker).
Financial weakness is often the culprit, a fading company gambling on flashy new styling to save the firm. Instances here (besides Cord) include the 1963 Studebaker Avanti and the 1951-54 Kaiser.
Now consider a counter-example, where nondescript design correlates to strong sales. Today's exhibit is shown below:
According to this Wikipedia entry, during its long (1982-1996) run, "the Cutlass Ciera was consistently Oldsmobile's best-selling model." Moreover, as another entry states, "Oldsmobile sales soared in the 1970s and 1980s (for an all-time high of 1,066,122 in 1985)," implying that the Cutlass Ciera was a popular car indeed.
The Cutlass Ciera design is not bad. I might characterize it as being competently mainstream with nothing to excite an automobile fan. It's the sort of appliance-on-wheels that a consumer advice magazine of that time might have approved of.
Let's put the Ciera design in Oldsmobile-historical context:
From the 1920s into the 1960s, General Motors marketing policy held to a continuum of car brands from low-priced to luxury. By 1941, the sequence from low-price to high was Chevrolet, Pontiac, Oldsmobile, Buick and Cadillac. There was price overlap between brands, but nevertheless it was a clear system whereby a satisfied buyer of, say, a Pontiac could "move up" to an Oldsmobile for his next purchase if he could afford to. The sales result was comparative stability amongst GM's brands during those times. The 1941 Oldsmobile shown above has fussy trim, but it sold well.
During the late 1940s Oldsmobile styling featured less ornamentation. The 98 pictured here was part of the top of the Oldsmobile line. I think it is an attractive design for its time.
Another clean design is this two-door hardtop from GM's portfolio of futuristic cars for the 1954 model year. The excitement dissipated in 1955 and 1956 as competitors launched their own versions of panoramic windshield styling. By the 1958 model year, Oldsmobiles featured Rococo styling in a misguided attempt to appear new and exciting. This was quickly reversed when Bill Mitchell replaced Harley Earl as GM's styling boss.
General Motors killed the Oldsmobile brand in 2004 because its declining market share could not support six different makes (GM introduced the Saturn brand for the 1990 model year). In the years leading up to this, GM marketers and brand managers agreed that Oldsmobile styling should be clean, with not much ornamentation. In the late 1990s, GM styling in general featured soft shapes and reduced trim, so Oldsmobiles became amongst the blandest of the bland, as this Intrigue demonstrates. It isn't a bad looking car, but it offers little visual excitement.
My answer to the question in this post's title must be inconclusive, so far as Oldsmobile is concerned. A stylistically unexciting Ciera was a good seller and the bland Intrigue marks the brand's dying days.