Mew Mew and its anime are intradiegetic or metaficcional (in other words, it's fiction that exists in a fictional world). It's Alphys's favorite anime in both games.
Mad Mew Mew is an animatronic possessed by Mad Dummy; the two things are separate.
We don't know enough about the franchise to make its own page. It's in the Media in Deltarune page, but I don't know about the Undertale Wiki.
Moved this to Wiki Discussion!
On topic, I'm only one person and I haven't prioritized writing that article at all, been too busy fixing up other articles for Deltarune.