Something I realized about the secret bosses
Each one is progressively less insane than the last.
Jevil: Actually clinically insane. He rejects reality and replaces it with his own.
Spamton: Deeply broken and barely coherent, but still more grounded in reality than Jevil.
ERAM: Some sort of malevolent entity, maybe even psychotic, but less noticeably unhinged than Jevil or Spamton. (I'd include the Knight here, but we don't know much about it yet.)
Gerson: A zany old man whose behavior seems odd sometimes, but it's usually to make a point (or because he's a bit of a troll).
Pink: Some lingering anger issues, but otherwise the most "normal" secret boss in terms of her behavior.
As for the reason... I don't think it's intentional per se; rather, I think Toby wanted each secret boss to feel unique, and that resulted in them getting a bit less Jevil-y over time.