Per the title, a crooner is a singer of popular songs or ballads usually with the accompaniment of a "big band" or a full orchestra. The crooner was popular in the 1920's, 1930's and 1940's with such singers as Bing Crosby and Rudy Vallee.
Once again in this episode, Pvt. Doberman alludes to being from Altoona, Pa., when he telephones his mother, Alice.