I don't think it's a BAD ending just kind of... bland. The antagonist that tried to drain magic from the world and reshape it as she liked just... walked away. No justice, no punishment, barely even a "No, that's bad!" level of scolding. They just fought briefly, her... daughter? Clone? Hugged her, and she just goes "Whoops, you got me! Guess I'll go to earth to watch soccer! Want some Oyakodon before I go?"
MC then goes back to stealing things with his harem, giving a veiled confession to one of them before saying "I'll go back home one day."
There's just no substance to it. Nothing really changed other than MC deciding to go home after he's had his fun, even though he has no real reason to.