He’s quite fascinated by astrology and the stars, even if he’s mildly disapproved of Mona. (Honestly, it’s probably because Diluc gets a little overwhelmed by her hyperactive attitude so he doesn't like her presence.) but he does enjoy looking at the constellations. He does, however, draws a line between blindly believing in supertistions and simply having faith. It’s quite literally difficult to NOT be at least a little supertistious when your world has floating islands and rumours of a cursed kingdom. But Diluc is very definitive about what he chooses to believe in that’s harmless and what is not harmless to pursue. He thinks the stars are beautiful but only knows they’re just dead planets residing far away. If someone tells him they’re souls of people who died then he’d just chuckle and go along with it - it’s harmless, he thinks, and as long as it makes them happy he isn’t going to spoil the mood.
Sometimes he might even try to convince himself to believe in the supernatural, since they are sometimes healthy for his heart - being too realistic will often send him into a depressing mood, and it’s good for his emotional levels to believe in something beautiful for once, even if they’re just temporary delusions.