Disney Wikia gives a statement what was the inspiration for designing Ducky Momo character. From Ducky Momo
Ducky Momo was directly inspired by writer Jon Colton Barry's younger sister, Kristin, who looks very cartoony/anime-like. She has extremely giant eyes and big, pouty, lips that give her a "duckfaced" appearance. In gradeschool children would often tease her and call her "fish-face" and "duck-face". She was also the inspiration behind the character "Meap", which is one of her nicknames and an inside joke between her and Barry.
This doesn't give citations but the creator of Ducky Momo, Jon Colton Barry replied to a tweet who he created that character. The tweet asks the same question.
@mmonogram @joncoltonbarry I found an assertion that Ducky Momo from Phineas and Ferb is inspired from your younger sister Kristin. Is it true? Do you have a younger sister? If not, what was inspiration for the character?
Jon Colton Barry replied
It's true. My sister looks a bit like Drew Barrymore and I designed Ducky Momo as a caricature of her facial features. The "Momo" name was inspired by something my friend Alicia accidentally said, which was "momo." To this day, no one knows why she said this.
Jon Colton Barry once again clarified on Twitter as a reply to Dan Povenmire:
Ducky Momo as a concept was not inspired by any specific other character/show, just sort of stylized Japanese animation in general. I designed his face as a caricature of my sister, who looks a bit like Drew Barrymore. His name came from something my friend Alicia said.
So, the inspiration of this character is not any other show but Japanese stylized animation in general but he drew Ducky Momo based on his sister's caricatures. His (Yes, Ducky Momo is a male) name came from Jon's friend Alicia's suggestion.