Answer:
No, Betty Boop was always a cartoon. She started out as a dog-like caricature of Helen Kane, a Broadway and Vaudville performer whose signature was "Boob-Boop-a-Boop" sung like a kind of scat in the songs she performed.
Later, Betty was humanized. Her long floppy ears were turned into human ears with long dangling hoops. Her features and conformation were also adapted to those of a human, based upon the appearance of Helen Kane (and/or Clara Bow).
Strangely, Betty's boyfriend, Bimbo, continued to be a dog, an anomaly similar to the fate of Goofy in the Disney cosmos.