I don't see how her being a sidekick prevents her from having a well done storyline. The difference between a sidekick and a co-star is the amount of episodes that focus solely on that character or where they play an important part. It just means that there will be less episodes for the sidekick and that she'll play a smaller part in the show, there's nothing wrong with that. Nobody is arguing against Iris not being a co-star, not being a coordinator or not getting 1 out every 3 episodes centered around her. What people are disappointed in is that the 1 out of 15 episodes that's centered around her just fails to deliver on the story part, which is especially surprising since all the elements were there for Iris to have a well constructed storyline. Kibago not being able to master Dragon Rage and Iris not understanding why, being frustrated by it... that could have been addressed, but it wasn't. Emonga disobeying her and not really caring about her, that could have been addressed, it wasn't. Iris' thinking that types are everything in battle and that no matter what strategy you use, you can't beat type advantage, that could have been adressed, it wasn't. Instead, after 27 episodes of being show that Iris is an inept and inexperienced trainer, she suddenly becomes Dragon Master extraordinaire Iris who doesn't have to work for anything, who never struggles, who's never wrong, who cannot be beaten by anyone other than Elite 4 members or tournament winners, who's uberknowledgeable and has magic powers. In the span of BW027, BW033 and BW042, they had pretty much taken away everything that could have possibly been interesting about Iris all the while painting Iris as a special snowflake who was right all along about everything. Even the Doryuuzu disobedience episode was just about Doryuuzu realizing that Iris had never stopped caring about him even though she was never portrayed as giving him a readon to do so, cause heaven forbid she would be portrayed as messing up or being flawed.
Being a sidekick doesn't exclude having interesting storylines that make a character progress in the few episodes that are dedicated to her.