Is it bad to be pregnant at 12?

Physically no, culturally it depends where you live. The youngest credible pregnancy that gave a natural birth instead of a c-section was 6 years old. By the time most girls are 8-12 they are fully capable of safely giving birth and generally recover faster from the after effects of giving birth then older adults do.

I would say yes, because your body is not fully developed. Pregnancy at age 12 has risks to both the fetus and the girl.