Is it a good decision to pursue MBA after completing B.Sc (Hons) in chemistry?

Any graduation will make you eligible for MBA provided you clear aptitude tests such as CAT/CMAT/MAT etc. as applicable to your institute of study.

B.Sc and MBA are not a good combination, but if you want to pursue MBA, you can do so. You need to be a graduate to pursue MBA.

No, MBA after B.Sc is not a good career choice. MBA is related to business management, and B.Sc is related to science. If you want to pursue MBA, you should pursue B.Tech first.