Yes, it is definitely a bumper for a 1957 DeSoto. Although there isn’t a lot of collector interest in the DeSoto, there was one model called the Adventurer that was interesting. In 1956, it was a special model that came in egg shell white with a gold band on each side. I believe that it was a special model in 1957 as well. Later model years, the Adventurer just became the top trim line. DeSoto sold moderately well in 1957, but it was downhill for the DeSoto after that. There were a few 1961 DeSotos produced before production ceased.
Chrysler products from 1957 through 1959 had severe rust issues. Your bumper would be important to someone restoring a DeSoto of this period.