Shaving a cat is good?

No, it's bad. Cats have whiskers all over their body which aides in their ability to balance and figure out if they can fit in small spaces.

It depends.
My parents have a very elderly 20 year old cat. He is long haired, and half persian, so he has a very thick double layered coat. In his old age he is unable to groom himself well, so my parents shave him every year in the places that he can't easily reach, so basically a lion cut. it was a bit of a struggle the first year, but he immediately felt better, and how he is happy to sit and get extra love and treats while my mom uses clippers and shaves him.

IF your cat does not need the excess hair removed in such a way, then it shouldn't be shaved.

yes it is good if you can do it without becoming a blood donor

No, it's criminal.