The games say that Daisy is a tomboy. However, I don't see her acting like one. Is there a good reason why she's called a tomboy?
Here's the background:
The Japanese instructions for Super Mario Land, the game where Daisy debuted, seem to be where this started, describing Daisy as:
(The important word here is おてんば, otenba, meaning "tomboy"/"tomboyish".)
Although the English instruction booklet for Super Mario Land has no such line, the English versions of more than ten other games (search here) describe her as one. Notably, this is what her trophy in Melee says about her:
The princess of Sarasaland, Daisy met Mario when he helped defeat the evil alien Tatanga in Super Mario Land. Princess Daisy is a bit of a tomboy when compared with her counterpart, Princess Peach. After her appearance in Mario Golf, some gossips started portraying her as Luigi's answer to Mario's Peach.
Daisy wears orange and Peach wears pink, but other than that I don't see Daisy as being less girly than Peach. Their dresses and hairstyles are both very feminine. You could also argue that Daisy gets kidnapped much less than Peach, but the first and only game Daisy was kidnapped in is the same one where she was first labeled a tomboy. (Daisy also isn't in as many games as Peach.) All of the other games I'm familiar with that have Daisy don't explore her as a character because they're either sports games or Mario Kart.
From the little I've read of different manga, I see it even less. (However, I am pretty sure that the games and everything else are different canons, and I am not aware of her being called a tomboy except in the games. The more information the merrier though.) In the Super Mario Land manga, Daisy gets kidnapped and Mario goes to rescue her. Peach wants to come along, but Mario tells her "no, it's too dangerous for girls" (i.e. だめだよ女の子には危険すぎる). What does Peach do? She tricks him by disguising herself as a robot and comes along anyway. (This manga can be found freely online in Japanese, but just be warned that it is sometimes NSFW.)
In a different manga (more specifically a 4-koma), it's Peach who's labeled おてんば (otenba, "tomboy"/"tomboyish") in the title: おてんばピーチ姫 (Otenba Pīchi-hime, "Tomboy Princess Peach"). Very little of this manga can be found freely online and it's also all in Japanese. But even from what little there is, it's pretty obvious how she's a tomboy (i.e. her actions). The Mario wiki describes part of it, and the (gross) image that goes with that can be found here.
Another example is these two pages:
- On the first page, Mario is nervous for his date with Peach. Peach gives Mario a present. Except the present turns out to be fire-spitting piranha plants who attack him.
- The second page right side has Peach asking Mario to close his eyes (so he thinks she's going to kiss him) and then she eats his food.
- On the left side of the second page, Peach sees a man giving his girlfriend his coat on TV and wants Mario to do the same for her. When he doesn't get the hint, she yanks his cape away from him — and to her surprise it is a magic cape and she goes flying.
Like I said I haven't read much (as I'm inconvenienced by not knowing Japanese), but there seems to be a better case for Peach being the tomboy rather than Daisy.