I can see an argument that illegal immigrants should not be counted because they are not supposed to be in the US?
In theory illegal immigrants are supposed to be arrested on the spot and deported when found out.
I can't see any argument to exclude legal immigrants from counting though.
The constitutional text itself requires that undocumented immigrants be counted. That's why "citizens" and "persons" both appear in the text. The tradition for over 200 years was to have them counted.
Well, you can be a legal resident without being a citizen