The incorrect statement about Cosmic Microwave Background (CMB) radiation is:
it appeared only a few years ago.
This statement is incorrect because CMB radiation originated shortly after the Big Bang, approximately 380,000 years after the event, not just a few years ago. The other statements are correct: CMB radiation does appear uniformly across the universe, its temperature is approximately uniform (with slight fluctuations), and it is indeed electromagnetic radiation that is a remnant of the Big Bang.