I have to say that MILF has to be older than 20. A 20-30 year old mum can be a "yummy mummy". It has to do with perspective:
1. Cougar is the older woman seeking younger men. It usually is not used for sps, I would think, because few sps advertise that they are only looking for young clients.
2. MILF is from the perspective of the client. It would mean she needs to be 35-45, usually, and remind him of the mother of his best friend when they were younger teenagers, say 14-17 (pre sexually active). She would be sexually attractive (but unavailable). A lot of mature sps like the MILF term, because it signifies a particular age range.
I don't think it is suitable for 20ish mums, because it is unlikely her children are old enough to have friends that find her sexually attractive lol. Using the Yummy Mummy title is to appeal to the friends of her SO lol, and more along the lines of Yes, I just had a baby, but I look damn good. Yummy mummy is best for sps who have kids under 5, imo.
I think all the terms are great, but there are certain expectations for some of them, and like GFE cause confusion when used out of context.