
It's not Bella's fault if S3 didn't go well as the first ones
In my opinion there were different reasons and choices that kinda ruined the show (Will's presence, tentacle's purpose, Emma missing and not talking about her at all, Zane and Rikki's downfall, a distant vibe from the original plot, static dialogues, unnecessary drama, etc.).
Sometimes people blame her personality not fitting with Rikki and Cleo and while it's true the OG trio is unbeatable, she would've matched a lot better with the girls if Emma was there too (I totally see the four mermaids being a strong and tight group).
Bella's character definitely deserved so much more expansion and she had the potential to explore the mermaid lore.
I guess we would've loved her even more if she appeared in S2 or from the beginning. Unfortunately they didn't give her a proper development but I'm glad to say that Bella almost saved the season, since she was the only one bringing a new energy into the story which wasn't necessarily a contrast to the first seasons.