At some point (1950s) the term ‘washerette’ also started being used for laundromats. However. . .
There was indeed a portable washing machine in the 1920s called “The Washerette.” Have not been able to identify who manufactured it, but the ads for this breadbox-sized doohickey tout:
The Washerette weighs but six pounds and may therefore be easily carried about. In fact, it is so small you can put it in your trunk. Just attach the hose to any faucet and the water will lubricate and run it. The result: Your finest garments will be washed spotlessly clean without the slightest wear on your clothes or effort on your part.
I also saw an add for a car dealership offering jobs to “washerettes” and “greaserettes,” who would respectively wash and lube vehicles. Those didn’t seem to catch on anywhere else, though.