Polling I saw says only about 18% of Americans are calling it that, with 72% sticking with the actual legal name (Department of Defense). Even a majority of Republicans are still calling it the Department of Defense.

The other name changes by the Trump administration are also not catching on.

70+% also continue to call the Gulf of Mexico "Gulf of Mexico".

A large majority also continue to call Mount Denali "Mount Denali".

A significant majority is still calling the Kennedy Center that instead of "The Donald J. Trump and the John F. Kennedy Memorial Center for the Performing Arts".