Is income in respect of a decedent taxable?

Yes. Death does not get you out of paying income taxes in the United States. I do several returns a year for deceased people. This is called a Decedents Return. If the person dies on January 1st they have to file a return for the entire year if they had income over the threshold to file. The Administrator or whomever is in charge of the Estate is responsible to file and pay the any taxes due from the Estate.