Is a curve in the penis bad?

No; mild curve is ok. A normal penis without peyronie's disease can be mildly curved when erect. Peyronie's is due to a thickened, firm area (caused by scarring after some inflammation or injury to the penis) in the compartment where blood flows in to pressurize the penis into an erection. Since the firm area (plaque) is a fixed shape and size, but the rest of the penis expands to get erect, one gets a crooked penis.
Depends. Most penises are not naturally 100% straight, and yet they are fully functional and cosmetically acceptable. There exists a disorder called peyronnes disease that is manifested by abnormal penile curvature caused by the development of abnormal tissue in the core of the penis causing a curve to appear. It is a progressing disease that if left untreated may significantly impair the shape & function.