Is pure corn starch effective in preventing armpits from smelling badly, or should I stick with baby powder?

? why not deodorant? If you have the androgen (hormonal) effect on your armpits that leads to body odor, a deodorant, with or without an anti-perspirant is the obvious solution. Corn starch will drop perceived moisture a bit but retain it & set up an interesting feeding table for body germs. Baby powder, (talc+perfume)) is less likely to feed germs but not likely to reduce body odor effectively., .