Happy Single lies in that unfortunate zone where it's not a good film, but also not bad enough to laugh at or hatewatch. It checks all the boxes of a romcom, the acting is passable and you'll find an occasional shred of humor. Overall, however, the main story thread is bland and the film doesn't lean into the absurd enough to become genuinely funny. It's a bummer because you can see a good sense of humor shining through the cracks.
The biggest issue, I'd say, is the main character herself, Molly. She's horribly unlikeable and while she could have worked as satire of self-obsessed millenials, it's abundantly clear that we're supposed to root for her as some sort of anti-hero. It's also difficult to buy into the relationship between Molly and her main love interest, Otis. Otis is a shredded, intelligent, empathic and charismatic chad who's single in his 30's (for reasons outside his control, obviously) and who's totally into Molly for...reasons. Maybe if the actors had great chemistry they could've sold this script, but they don't so the entire thing is really difficult to buy into.
In summary, this isn't the worst Dutch movie ever made, but it's also definitely not good enough to recommend you watching it.