The “is a mother” requirement to me is nessecary to be a MILF. But, it’s open to interpretation because a woman doesn’t necessarily need to have biological kids of her own. Raikou from F/GO is a an example of that; she’s the “mother” to Kintoki.

Personality (outside of saying they should have a motherly aspect) really doesn’t work much either because there are a lot of different MILFs out there. Izumi from F/MA definitely isn’t your ideal mother-archetype but she’s undoubtedly a MILF. And Raikou is a yandere mom.

Also, just to mention a mom that doesn’t fall into the big-titted variety: Mavis from Hotel Transylvania.

Of course there is always the “What’s a MILF to you isn’t a MILF to someone else” aspect if going by the literal definition of the term “Mother I’d like to Fuck”.