During Thanos's snap, which was supposed to wipe away half the population, also all clothes were dusted away, including Bucky's arm, even though the gun he was holding at the time didn't dust away. Also the thing that Nick Fury was holding to call Captain Marvel didn't dust away. Why's that? Didn't Thanos only mean to dust the actual people?
Whatever item is considered part of the person’s identity seems to get Snapped with them.
According to Joe Russo, the objects faded with the heroes and other victims of Thanos erasure because they are "part of their identity."
Anthony Russo chimed in: "Whatever was elemental to somebody’s presence went with them."
The Snap is an interesting idea of Thanos that he thought through. His purpose is to save or relieve the universe of half of its population because overpopulation is causing so many problems. So in his mind, he is the good guy.
With that mindset, he would not want to cause problems for the people left behind. The other half. That is why people turn into dust instead of simply dropping dead - then you would have a lot of dead bodies, with all the consequences (diseases, cleanup work, aesthetics, etc.)
Leaving clothing behind would be a similar issue, on a smaller scale. There would be empty clothing everywhere, causing problems and inconveniences. Removing the half he is removing entirely - clothing and all - is simply cleaner, more beautiful in a way, in his twisted mind.