Just want to add to this:
They are two nitpicks I have with this episode. The first one is a "minor" one, about the Remoraid machine things. That was definitely a not very creative way to force Piplup to have to use its contest bubblebeam. Remoraid was a nice choice, in terms of old pokemon showing up, I guess, so it's not all bad. The second thing would be the way Ash doesn't call out Starly or Turtwig or Aipom to help Piplup when it was struggling. So this whole scenario wasn't too well thought out.
But what I really liked was the way Ash, Brock, and Dawn observed what Piplup was doing, rather than intervene directly. I think that's the type of interaction that defines Sinnoh. You never get the sense that our heroes know exactly what the right thing to do is, and, really what exactly is the right thing to do isn't clear either. That's something I've always really liked about the Sinnoh episodes, b/c I believe real life is like that. Of course, many people don't prefer to observe and don't hesitate to just jump in there and take action, but I'm just not one of those people. So for me, I have an easier time relating to the Sinnoh episodes.
Every now and then, a topic will open up about the group dynamics of the Sinnoh group, and I think this epiosde was really good in showcasing the way it normally works. Though this is Dawn's home region, she's the "stranger" in this group. Ash and Brock have traveled together for so long and are pretty good friends. Dawn is also more hesitant to voice her opinion, mostly saying what she thinks in a questioning voice, b/c of her inexperience as a trainer compared to the relative experience of Ash and Brock. The differences in experience allow for variety in the storylines; Ash's storyline, in terms of the skill he has to show in order to overcome his obstacles in Sinnoh, is much more complex than Dawn's. You can see an example of that here. Last episode, Ash lost a very tough, tough match to Paul; here, Dawn gets character growth just by believing in Piplup. But to make up for that "simplicity," if you will, Dawn's journey is much more emotional; and it's fun to watch. I think one thing that's lacking next region is that all three characters of the group feel equal; they are the same, and there's no real variety, in terms of turmoil, of their journey (subjects are different, yet complexity of solutions and how the problems are solved feel similar).