When Mob exorcises Dimple for the first time, he taps his fingers on Dimple's forehead and says: 僕は最低だ。That gets translated "I'm terrible" or "I'm the worst" in English.
Those are pretty vague words in English, though, so I paused the show to look it up. (took me a hot minute to find the listing for the word 最低, too, which means I'll probably remember it forever, but that's not relevant, lol.) SO. Mob says "boku wa saitei da"— I am 最低. Turns out that "the worst" is a pretty great translation, because it captures both the aspect of "least (lowest in some category)" and "most repugnant" from the original word.
This is awful. Obviously. No 14-year-old should be calling himself that in full honesty. But that's not the point I want to get across:
The exact moment that Mob uses the word 最低 for himself is worth noting! Dimple has just jumped at Mob for a killing blow, proclaiming that in the battle between them, lived battle experience will determine who's the winner. Dimple is judging himself and Mob against each other on a scale of battle skill. The only variable he cares about is power. And when Mob wipes him out, clearly Mob is the winner of their contest. From Dimple's perspective, Mob should be saying he's 最高—saikou, the antonym of saitei, meaning "the best/highest". But Mob is judging himself by a completely different variable—the ability to control himself. That's why he calls himself the worst. It's a moment of self-loathing, yes, but not out of nowhere—he's correcting Dimple and re-contextualizing their fight as a failure on his part, not a victory for anyone.
[I.D A digital fanart of Katsuya Serizawa from Mob Psycho 100. He is drawn in a top down perspective on a blue background. A transparent umbrella with its colourful highlights obscures the top part of his face. The rain left some droplets on the umbrella. The shadow under Serizawa's feet is purple. End I.D]
ive always felt that serizawa actually doesnt know a whole lot when it comes to spirits and curses. in claw, he mostly dealt with other psychics, and before that he was completely isolated. the supernatural has always been present in his life, but he has no real knowledge about it aside from educated guesses mid-exorcism and stuff he picked up from video games—he was actually surprised that psychic powers worked on ghosts at all, since in pokémon psychic types are weak to ghosts and not the other way around.