"Dreamgirls" was originally a Broadway musical, and the film retains about thirty songs or song snippets from the original stage musical, with four new songs ("Love You I Do", "Patience", "Perfect World", and "Listen"). Since the film is a musical based on a Broadway production, and, again, is not actually a Motown/Supremes biopic, there is no need to use Motown songs or imitations of such. The only song that could be labeled an imitation is "Perfect World"; however, that song is presented as a parody of the Jackson 5's hits more than anything else.
However, a number of the songs do pay homage to the Supremes and the other R&B girl groups of the day (for example, the instrumentation for the song "Dreamgirls" is rearranged from its original Broadway orchestration to sound more similar to a Motown Funk Brothers band track, and the distinct influence of The Supremes' styles of singing and harmony can be distinctly heard on most of the Dreams' songs). The "Dreamgirls" film also takes inspiration from sources outside of Motown: strains of James Brown, Jackie Wilson, Little Richard, Etta James, Aretha Franklin, and Donny Hathaway can be distinctly heard throughout the film.
Incidentally, if "Dreamgirls" were actually a Motown biopic, only a handful of its songs should actually have the Motown sound, in order to be historically accurate: "Love You I Do", "Dreamgirls", "Heavy", and the "I'm Somebody" medley. The other songs are either (a) performed before 1963, before the Motown Sound had fully developed and the company's releases were closer in style to the other R&B/soul releases of day, (b) performed in a venue for which Motown would have rearranged its songs to make them more palatable to a particular audience (i.e. Las Vegas), or (c) performed after 1973, by which time the Motown Sound was becoming less distinct again, due to the changes in the soul music industry at the time.