There is a huger diffrent in frequency. Microwaves are at 2.4 GHz but the CMB peek at 160gGHz
The heating effect is one of intensity. Compare it to visible light that is just electromagnetic waves like microwaves. The light from a small flashlight on your skin does not heat up you a lot compared to sunlight. Sunlight heat you up less than sunlight concentrated with a magnifying glass which will result in burn damage in a few seconds.
The CMB will deliver less power per area unit than a small flashlight and microwave that have a higher power density than sunlight.
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