>>45342934It's rounded edges and cutesy appearance make it appealing as a starter Pokemon in the video games, as well as a fun toy or plushie for children.
In the Pokemon games, Piplup's final evolution is Empoleon, a Pokemon with very well-rounded stats suitable for almost any playthrough of the game (as is the case with many starter Pokemon). It has relatively high Sp. Attack and Sp. Defense stats, and it's Steel-typing gives it many resistances to other types in the game.
In the Pokemon anime, one of the main characters, Dawn, owns a Piplup. Piplup was most likely chosen because of its cute and appealing design, as well as looking a tad dopey. Because of this, it works very well as the comic relief character.
In conclusion, Piplup was designed with cuteness in mind to be both desirable to new Pokemon trainers and to sell products. It's final evolution stage is a reward to those who took the time to train it, and Piplup gets lots of screen time in the Pokemon anime.
