The design of the throne was definitely a group effort. The Return of the Jedi Sketchbook shows a selection of concept drawings for the throne room on the Death Star, including an interesting example that shows the throne suspended from the ceiling.
The black-and-white concept art in the book was produced by Joe Johnson, Nilo Rodis-Jamero, Ralph McQuarrie, and Norman Reynolds. As you can see from the differently styled sketches of the throne, more than just one person contributed to the design process. The throne suspended from the ceiling is the work of Johnson, the art director—visual effects (identifiable by his handwriting on the comment). From the style of the drawing (especially the shading), the other concepts on that page are identifiable as the work of Rodis-Jamero, the assistant art director.
There was color artwork by McQuarrie (conceptual artist), bit it came at a rather late stage of the design process, since it shows the spoked window ("a fitting background for the supreme ruler of the galaxy," as the Sketchbook comments put it).
The general shape of the throne and the design of the surrounding room were fairly well developed before that element was added.
However, McQuarrie did also contribute to the design process, as did Reynolds (production designer); I particularly like this unused throne/throne room design by Reynolds, with the twisting walkways.
(A scanned PDF of the Sketchbook can be found online, but I'm not going to link to it. I actually own a physical copy of the book from 1983, and it's a wonderful resource.)