Could microwaves passed through the head read what a person is thinking? · Grey Matter

With the physics as it is known, no.

Could microwaves passed through the head read what a person is thinking?

With the physics as it is known, no. Microwave tomography maps how much water and salt tissue holds, at grids of a few millimetres and wavelengths of centimetres inside the head, which is enough to find a pool of blood after a stroke and far too coarse and slow to follow neural activity, whose effect on those properties is tiny. A worry about mental privacy is better spent on the methods that do read activity, such as EEG headbands, fMRI decoders and implants, where the data already exist.