Not true at all about Ponyo! I just got back from watching the movie and I'm thinking about cosplaying her myself. And despite it being said that she is red, it's more of a pink color than anything. Nor is her hair curly or red either, it's a pinkish-red.
Like any costume, start with fabric and patterns! Look at patterns that look like Ponyo's dress (I highly suggest taking a few reference pictures with you to the fabric store!) Look for patterns for both the dress and the bloomers! I'd start with making the bloomers. If you've never sewn before, bloomers are a very simple thing to start off with making! I'd suggest a thicker fabric for the bloomers to help give it more of a poof!
A great thing about Ponyo's dress is there are shorter 'loli-styled' dresses in the costume section of almost all pattern books that have the poofy sleeves! After the dress is made, I'd hem it to a good point where your bloomers will still show like hers do!
I hope that helps at least a bit!