In current Gen Z slang, mother is a queer-coded ultimate compliment for a powerful, glamorous woman, especially a pop star. "Beyoncé is mother" means she is iconic. Roots in Black ballroom culture, where "house mother" was a leader. Not concerning. Used by all genders. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.