Daisy is in my opinion, several Smash Bros. games from being a playable character. Maybe the 6th or 7th Smash Bros. games in the series she'll make it. She's quite qualified to be an assist trophy, though. Maybe she could be similar to Waluigi by using a sports item like a baseball bat or tennis racket.
The reason she should not be a character is obvious. We really don't need a clone of Princess Peach, plus you can pretty much play as Daisy just for the fact that one of Peach's alternate costumes has been the exact outfit Daisy wears. In short, she should appear, but not as a character.