Thematically perhaps, but the question is whether he has a direct connection to them and Pecharunt as you can have a character parallel a story without directly following it beat for beat.
I can't say I agree it was sudden, at least not in the context of how short the Teal Mask arc was. We have a clear origin for when Kieran starts acting differently as he overhears the player, Carmine, and his grandpa discussing about keeping Ogerpon a secret and then on he starts spiraling. The player that he started to trust and his family lied to him, justified or not, and is keeping something he adores away from him as he loves the Ogre and has idolized it for years, that's of course going to bother him and make him less kind to the player. From there it gets worse, he never beats us despite changing around his team and trying to get stronger, he tries to win Ogerpon's affection and instead she attaches to the player, and he sees the player cozy up to his sister who was abrasive to them before the whole story went down. I can understand that it could've been stretched out more, but I don't think it was like a switch went off with the character, it felt more like a progression (or rather, regression) to what Kieran became in the Indigo Disk.
And I think another problem with the theory is that it sets up Pecharunt as a big grand schemer behind every action he takes but the story limits that. For instance, you mentioned that part of the reason Pecharunt corrupted Kieran was to get Terapagos and the Tera Crystals, however, nothing indicates that Pecharunt was aware of Terapagos. Kieran doesn't know about it at all until Briar talks to the group after he loses to the player, and he's surprised to learn about it, so he couldn't have been fed any information about it from Pecharunt. Additionally, at that point Kieran has a very personal reason to get Terapagos since it'd be a way to potentially one up the player, so it's not like Pecharunt's influence would've changed the action. For the other bits about the Pokemon, the backstory only sticks to it and the Loyal Three trying to steal the masks because they were beautiful masks to steal, the crystals weren't given as the reason (or at least the only one) for why they went for them. The event story does leave its motivations for controlling the town ambiguous, but you can use Terapagos against Pecharunt and unlike Ogerpon or the Loyal Three there's no reaction between the two which implies it's not something it's invested in. Nothing indicates it knows or cares about Terapagos, so it doesn't really lend to the idea that it manipulated Kieran to go after it, whereas the story gives Kieran good reason to do it for himself.
In general though, I feel the problem with the theory is that it's assuming Kieran's character arc wouldn't have happened without Pecharunt's influence, which I don't agree. Kieran has good reason to start his arc as the player (and his family) give him reason to be upset, he starts getting envious of the player because the player has multiple good things line up for them (including getting Ogerpon) while he's just a kid who feels he can never measure up, and even after getting stronger he still loses to the player and in a desperate attempt goes after a Legendary and screws up, forcing him to finally get out of his funk. The arc has a flow that doesn't need Pecharunt's influence to happen, and there's little that a possession angle would add to it.