To be fair, I don't think that making a game for yourself is strictly bad, the biggest problem is Velminth's reaction to criticism, if I have to single out a reason.
I have two examples of people making a game (and a porn game, at that) for themselves and not being fucking assholes about it:the first is
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by mminitdev and the second is
Some Heroines Climb Up a Tower to Ask God Why the Game Has a Name That's So Long by Sinful Salad Games.
In the first example the author is straight up just making a porn game driven by passion, as the only way to give him money is by a donation that itch.io allows, but only if you want to, because the game is free. And it's his second game, with his first one being made the same way.
In the second example, while the creators are accepting money via patreon, they're completely honest about what they're doing and why the're doing.
Both examples are united by a rather simple thing: they are not throwing tantrums the second they hear criticism about their games. Like, I fucking hate rogue-likes and I wish "Some Heroines" was a rogue-
lite instead, but I wouldn't dare say that it's a bad game. You can check the dev's responses of both examples, if you want, and you'll notice that they respond in a respectful manner and don't try to belittle critics, unlike Velminth who straight up called people slurs when people criticized the card mechanics.