He turned professional, big-time, in 1913, signing with the New York Giants baseball team, the Yankees of their day. After a rocky few years playing the one sport that did not come easy to him, by 1919 he was hitting as well as Ty Cobb and Joe Jackson.
Ty Cobb 1919 oWAR (offensive wins above replacement): 5.3
Joe Jackson 1919 oWAR: 5.6
Jim Thorpe 1919 oWAR: 1.0
Even relying on basic stats Thorpe comes us short; he hit an empty .327 in 1919. Jackson hit .351 and Cobb hit .384, both with power and walks. For his career, Thorpe was .4 wins below replacement. Jim Thorpe was indeed a fabulous, extraordinary athlete, and it’s not likely that many Olympic decathletes could turn in a baseball career that came anywhere near replacement level. This is rather a different thing than suggesting that Jim Thorpe was a good baseball player. FWIW, both Bo Jackson and Deion Sanders were considerably better baseball players than Thorpe.