Chris Sutton, vice president of automotive retail at J.D. Power, called the phenomenon "the Amazon effect."
He said the desire for "seeing, buying and having a product delivered to your doorstep has made its way into vehicle-buying, and it is here to stay."
In the closely watched annual ranking, three General Motors brands took the top three positions in satisfaction among mass-market brands.
GMC ranked highest, displacing Mini, which held the top spot last year. GMC rose from No. 2 last year with an 8-point improvement, while Mini experienced a 30-point decline, tumbling to No. 6.
Buick rose one spot to finish second this year, and Chevrolet ranked third, followed by Dodge and Nissan.
The mass-market average inched up a point from last year, to 785 on the 1,000-point scale.
Brands showing the greatest improvement from 2020 were Jeep, with a gain of 24 points; Chevrolet, up 17 points; Dodge, also up by 17; and Ram, with an increase of 14.
J.D. Power calculates the brands' scores by factoring in what customers say about the delivery process, dealership personnel, negotiation, paperwork completion, perceptions of the facility and their satisfaction with the dealership website.
The study surveyed 35,387 buyers who purchased or leased a new vehicle from March through May.
Porsche, which has made a concerted push into online selling, was the highest-scoring luxury brand. The German sports car maker rose five spots from a year earlier. Infiniti ranked second among luxury brands, followed by Lexus and Cadillac.
But satisfaction with luxury brands on average dropped by 9 points year over year, to 807.