Far as I can recall the Constitution doesn't apportion representatives based on number of citizens, anyway.
House of representatives is proportional by population. In theory. In practice a state with a really tiny population would get disproportionate representation but I don't know if any are that small.
Again, the 3/5 compromise was supposed to deal with the "Citizenship" (or rather, 'right to vote') question. They still counted, but only at 3/5 the value of a person with bonafide right to vote. (Remember, women also counted in the 3/5 compromise, because they could not vote.) Instead of asking for citizenship, they prejudiced based on gender and race. We still ask gender and race, but those things have no bearing on right to vote status. Citizenship does.
Again, I DO NOT SUPPORT the GOP's bullshit.
However, keeping fucking Russia from sending a shitload of "Tourists" during census and election seasons to fuck with our electorial processes seems sensible, given that they have been meddling in it in all kinds of other ways already. Opening the door to noncitizens enables that kind of bullshit.
I should probably further clarify-- 3/5 compromise is NOT IDEAL. This is not meant to read as an endorsement; Rather, only a reminder that while in the past the topic of citizenship did not appear in the process, it was because the function was performed in other ways, which the 3/5 compromise embodied.
We do need some form of protection in this area, just what a suitable one would be, I have no real clue. Only that "Does not matter!" is dangerous levels of stupid.