If the story of Reborn is not your cup of tea, it's also an option to not play the game, or just ignore it and play for the gameplay. It's also an option to focus on the more positive parts of the game, rather than just complaining about the "bad things" all the time.
And as there's people who do not like the story, there's also an overwhelming amount of people who DO like the story! Evidence of this is basically how big of a community this forum has, and how many people appreciate Ame and her dev team's work.
And let's be real here about the characters in terms of design choices; would a super happy looking protagonist with incredibly bright colors fit that well as a silent protagonist in a (semi) post-apocalyptic setting? I wouldn't really be all that content either if the train you were on would get blown up upon arriving in the city while you didn't ask for that to happen at all. Giving too much personality to a character based on their sprites is also a bit dangerous as the player is supposed to be able to put themselves in the silent player character's shoes, which would lead to more of a neutral color scheme. Given the setting and the situation the player character is in, the designs make a lot more sense than if there was some kind of super bright and colorful player character. It's simply more fitting.
And in regards of angsty and edgy characters; seeing as a lot of bad things happened before your arrival, I wouldn't be too happy either. Sure, things are a bit angsty and edgy because of it, but again, taking the setting of the game in mind, it's all pretty justified as of why the characters are like that. Of course, they look a lot brighter than your player character, but a lot of these characters are gym leaders or other major plot characters, and having them ALL blend in with the environment would make the game look a bit bland.
In any case, I hope it explains a few things. Please know that these are just assumptions in regards of the design choices as I'm not part of the dev team but rather looking at it from a game dev's viewpoint, but it'd make sense if those were some of the reasons behind the choices made. Reborn has its flaws, but it's really not as bad as you're making it out to be. A lot of game design choices are justified by the overall setting and feeling of the game. If I would go into this any further though, it'd become Opinion Town™ so I won't do that.