Mickey Rooney (born 1920) is an American actor who has been working since 1922. He has worked in film, television and on the stage, and has won an honorary Oscar, a Golden Globe, and an Emmy.
As for his appearance in Gravity's Rainbow, Rooney was indeed in Germany with the Army at the time of the Potsdam Conference, but was unable to attend and meet Truman himself. However, Rooney's presence may be a reference to Hello Pal Comics. This series lasted for only three issues, but featured a character called Rocketman, as well as a photo of a movie star in each issue. The first issue featured Rooney's image on its cover. So, Rocketman and Mickey Rooney had already crossed paths before Pynchon brought them back together.