Why is "to" not accepted here?
'Moja mama to aktorka' should be accepted.
nope, it's not
Added now. But I don't recommend such construction (X to Y) if X is a person, they seem a bit clumsy.
And they are just too clumsy to accept if X is a personal pronoun.
moja matka jest aktorka, should be accepted in this case as well (according to a native Polish speaker).
"matka" is much more formal than "mama" (and "mom"). We only accept it when the English sentence had "mother" and when the sentence is about "being a mom/mother" generally. Not for "my mom".