You have to get all the X marks with "Good" in order to get a "COOL" score. Get some "Excellent" marks and you'll eventually reach "FANTASTIC" ;P
I'm the master of Ice Cream Beat but only because in my first BBS run I practiced a lot. And I mean A LOT. Yes, it helps if you dance to the music to the left and to the right, much like the duck nephews, but my trick was pressing every button like actually hitting it ("BAM BAM BAM" xD) and also, during the Combo move, putting the PSP on my knee and pressing the hell outta the X button like that. Years passed and I replayed Aqua's story recently, and I got FANTASTIC scores right from the first try. You just gotta feel the songs and love them, my fav is "Blast Away!-Gummi Ship II-" because I can totally dance to the fast rhythm of this one xD
another tip: control yourself. By this I mean do not fall for the urge to press a button as quickly as possible, you'll catch the note anyway, risk a bit and wait till the circle is really close enough for an "Excellent" hit! This is what I practiced, self-control
like, this is one serious mini-game, y'know? I sounded way too serious