"My mom is an actress."

Translation: Moja mama jest aktorką.

[Xelareip]

Why is "to" not accepted here?

[luless]

'Moja mama to aktorka' should be accepted.

[Bejay13]

nope, it's not

[Jellei]

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.

[karel792398]

moja matka jest aktorka, should be accepted in this case as well (according to a native Polish speaker).

[Jellei]

"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".