Rebeca Dart (Art director) and the lead VFX artist (I forget his name) have claimed on Twitter that the animation in the movie is not indicative of the animation in the final product. I would imagine the trailer was compiled by an alternate studio.
Onto the aesthetics as shown - presuming this is an accurate reflection of the art style - it's certainly more rounded and soft, which I think adds to the already highly neotenous features of the characters. I would say they appear more cute, but as for how this single variable will affect the overall mood of the work, I would imagine not nearly as much as other aspects such as writing and direction. Luckily, Meghan and Jayson are doing just this. Considering they have been working on the series since the very beginning, and in my opinion have a solid grasp on the general air of the series, who better to be in these particular roles.