It is a movie title. Grammatical rules may not apply or be bent. We also have an expression "Shall I be mother?" This is used on the UK side of the pond. It relates to when people are sitting down for tea and the tea is in a teapot. By tradition the oldest person present or the nearest person present picks up the teapot and says "Shall I be mother?". This will be the person who pours the tea into the cups for everyone.

Thank u Ron and Michael Smith. Really useful. 🌹

I've also seen this movie. The robot is the girl's and everyone else's "mother", so she's the only one. I think there is a God reference in there as well somewhere (I am God), but I'm not sure. If my children were to ask me - for some reason - who I am, I'd say "I am dad". If I wanted to state to others that I have children, I would say "I am a dad".

Yes. it's a non-standard usage, but probably quite deliberate (and is not wrong). Without the article, on its own, "I am mother" suggests a different meaning : as archetypal mother. There was a song like this, called "I am woman". https://youtu.be/V6fHTyVmYp4 https://www.educationtopia.net/grammar/nouns-used-without-articles