I'm probably just weird, but I'd like to see Dawn catch a Trubbish. She did a great job working with the Grimer at the summer school, so I think she has the skills to take an ugly-cute pokemon and really bring out its appealing side. Plus there'd be huge potential for humor there. Just picture it, Dawn and her lovely trash bag.
Did Iris really use Dawn's catchphrase? I don't remember that. If she actually says "No need to worry," that would feel pretty wrong. But if she just says "daijoubu" in Japanese but says something different in the dub, I don't think that's so bad. Daijoubu is actually a really common word. What makes it special for Dawn isn't that she uses it, but the way she uses it. If that makes any sense.