Oh you're right I didn't think of it that way. The thought process was that since the legend its based on is about not giving up, the health bar should be motivation. I was gonna make the currency for upgrades something abstract like believing in yourself or something but my partner said "Koins" and I couldn't resist the pun lol.
I don't think its about trying to avoid ridicule though. The fish wants to get to the top of the river despite the ridicule. It's never implied that they're trying to prove themselves to anyone. The demons are literal demons from Japanese/Chinese myth. I think they're meant to represent people mocking you when you try to achieve the "impossible". And the grind isn't endless, the game does have an end and you can win. Still, I get how your interpretation makes sense, and yeah it is kinda fucked up :/