Since the census is used to apportion representation in Congress, and since only citizens are legally authorized to vote for those representatives, being able to count actual citizens, separate from illegals, sounds like a good idea to me.
Might seem like a good idea, but it would not really be following the Constitution. At the time it was adopted, women, slaves, many men, & children could not vote, yet they were still counted (if only 3/5s in the case of slaves) to apportion representation. To count only citizens for representation would require an amendment, I think.