Portman was a brilliant man, and a real artist. He got to work on a grand scale, and was up to the task. I don't know that his ethos began as "populist," but many folks who knew nothing of Modernism first experienced the movement in his epic hotel spaces and were moved and inspired by them.
I recall a quote from Julius Shulman's agent, Craig Krull, at the time of the great photographer's passing:
"Modernism is characterized by an optimistic spirit, a belief that the future holds great promise and technology will improve civilization."
John Portman's work embodies that optimism.