I think that Daisy was the old Peach, or we can say "Princess Toadstool"! XD In fact, the pink princess didn't have a real name those times yet, so maybe they created Daisy, who maybe was the Princess Toadstol with a new look and a name (as shape they looked the same, eccep the colours). So maybe she was created as the princess-love of Mario, but after Super Mario 64, when Peach got her real name, we could see Princess Daisy as second princess in Mario Tennis 64 if I don't remember well. I remind you that Super Mario Land was created by Gumpei Yokoi (correct me, if I am wrong), and not by Miyamoto, so maybe the Gumpei wanted to give himself a name and a new outfit for the classical princess...