There's no reason it should be, it's unclear to me which scenario concerns you:
Mirrorless cameras can be powered by USB, either Micro or C depending on model. The batteries have a built-in BMS so leaving the camera plugged in won't harm them.
Some mirrorless aren't thermally compatible with running full video on battery for long periods, this is worth researching before purchase, but for the ones which are it's not going to damage the battery to run off of it.
Mirrorless cameras can be powered by USB, either Micro or C depending on model. The batteries have a built-in BMS so leaving the camera plugged in won't harm them.
Some mirrorless aren't thermally compatible with running full video on battery for long periods, this is worth researching before purchase, but for the ones which are it's not going to damage the battery to run off of it.
There's no motive to do so with a desktop setup.