Dawn's cameo was full of wasted potential, which was quite unfortunate since it does feel like her cameo didn't have any significant impact and she did seem like an afterthought during most of the Junior Cup episodes. For starters, I would have have her show the ribbons she won in the Hoenn region thus far and then explain that she came to Nimbasa City to gain inspiration after losing the last couple of Contests. That would be similar to why May decided to compete in the Wallace Cup, but at least it would make more sense as to why she suddenly jumped from Hoenn to Unova and Dawn needing some unique form of inspiration would have been believable. Ash asking about how that modeling offer went would have been nice, but not as necessary as seeing how her Contests went. I also would have changed her team a bit more beyond just having Quilava. Even if they didn't end up doing much, like most of Dawn's team did anyway, it could have shown that she had been making more progress through her journey than basically have the same team for the most part. So either having Hoenn Pokemon, Bunery evolving, as I think that should have happened back in DP, or capturing a Unova Pokemon would have worked for me.
I also would have had her interact with Ash more since they didn't really feel as close as they did back in DP, especially after watching the finale where both she and Piplup were quite sad about leaving the group and going on their own. Maybe more training with them and not trying to make Dawn and Iris look like instant BFFs would have helped. I would have toned down Dawn's interactions with Iris since it did come off as a generic attempt at making them look like good friends right off the bat, rather than having them genuinely bond.
I also would have had Dawn defeat Iris in the Junior Cup. The Junior Cup would have been the first opportunity for Dawn to showcase what she had learned in Hoenn and Unova since she didn't want to just wait until she returned to Hoenn, or go for the Wallace Cup again, to put into practice what she learned. Dawn would have used her Contest skills in that battle, such as that Ice Armor bit and other ways of combining Mamoswine's attacks, to throw Dragonite off its game since it was clearly more familiar with head-to-head battling. It would have shown that Mamoswine and Dawn were in sync and that along with Dawn's Contest combinations would have given her an edge over Iris and Dragonite who weren't able to work together. Dawn could have explained to Iris that she and Mamoswine didn't get along at first either. Instead of just saying that to tell Iris that it will all be okay eventually, she would tell Iris the story in more detail and tell her how she got to be friends with Mamoswine. That would have Iris realize that she needs to work on her relationship with Dragonite off the battle field, even though that should have been obvious to her anyway, and would have been a better way to show the two of them being good friends than having them instantly click. Not to mention it actually would have a more significant impact on Iris's storyline than giving her another undeserved victory. This would mean that Dawn would lose to most likely Trip, which wouldn't be satisfying, but at least she would have already had more impact on the Junior Cup than she did already.
I would have been fine with Dawn still going for the next Wallace Cup if she mentioned how her journey through Hoenn was going when she first arrived. Dawn defeating Ash in their battle would have been nice too, or at least ending it in a tie since it was a cool looking match and just having Cynthia end it wasn't a good way to end it. It was so disappointing to see that her cameo had no significant impact compared to May and even Misty's returns. Even if it was a sudden change just for tradition and bringing back older Pokemon like Piplup to promote again, just a bit more effort put into it could have made this cameo actually mean something significant.