It really depends on the particular manga. Some depict Mash having the ability to use magic, while others might not. It all comes down to the creator's imagination and the story's requirements.
She might be initially stunned. After all, Percy is a hero and his well - defined muscles could be a sign of his strength and capabilities. It could make her blush a bit, secretly admiring his physical form while also feeling a bit flustered as she tries to maintain her usual composed self.