The pink color is the determining factor in making women prefer the character. I have tried asking why women are all addicted to the color pink but till data, I'm yet to get any distinctive explanation to why such habit or behavior exists in all woman. Seriously, I don't think that I have seen any girl who doesn't love pink color, they all seem to love all their dresses to be pink in color, toys pink in color, shoes pink in color etc.
Some adult females are still having such preference with color when it comes to choosing what they want. I can remember vividly during my university days in the university, most of the girls I visited in their hostel all had their rooms painted in pink color. There was this particular girl that makes it a duty to repaint her room every 5 months. So, it's probably their likeness for pink color that makes them love Mewtwo.