There really isn't an "ending." The MMC is reluctant to go out with the FMC at first, but warms up to her over time. They start dating. They work together at the school festival. The FMC says she wishes they could have more days like that. The MMC agrees, and thinks to himself how he knows for sure now how he feels about her. That's the end of the oneshot, but it's obvious that if the manga were to continue they'd continue to develop their relationship as they're both in love with the other.
Can't say that's a bad ending. Maybe it doesn't offer you complete closure, but there's no unresolved plot threads or disappointed rivals, so it's a good end in my book.