Questions & Answers about Soy un poco torpe con las tijeras.
Because torpe here describes a general trait or tendency, not a temporary state at this moment.
- Soy torpe con las tijeras = I’m clumsy / not very good with scissors in general.
- Estoy torpe usually means something more temporary, like I’m being clumsy today or I’m off my game right now.
So ser is the natural choice when talking about someone’s usual level of skill or ability.