I feel like it would be better to phrase that as you found them uninteresting.
Or did you mean they are un-interesting in a way most of society would say someone is uninteresting. Like they aren’t interesting because they don’t follow sports or TV shows? Or they only talk about work or kids? Those people can still be interesting, just to different types of people.
They may not be interested in being interesting for other people?
Dog smoking in a bath