It seems very odd to me that they keep adding these rarity grown characters that are stronger than a rainbow itself.
I mean yeah, Rarity growth takes a long time to accomplish, but the fact about it is that rarity growth is potentially more common than actually drawing a rainbow unit....
One could argue that the dress slot is a problem, but considering how lucky getting a rainbow pot is, getting the same rainbow on DMM at least has only happened once for me in the 2+ years I have played.

Regardless, back on topic. I wouldn't overthink this deal. Nutaku pricing is all over the place and honestly they probably could care less about mum. She is a fairly weak unit whose collection value was already taken advantage of.