Krishna addresses Arjuna directly: O best of Bharatas, know that any sacrifice performed with an eye on the reward, or for the sake of impressing others, is rajasic.
The rajasic worshipper's motivation is not pure. They may seek wealth, fame, children, or status. Or they may want the community to see them as a great devotee. The ritual is performed not for its own sake but as a transaction — I give this, so the universe owes me that.
Such a sacrifice is not entirely wasted. It is still a sacrifice. But it falls short of the sattvic standard because the motive is contaminated. As long as the desire for reward or recognition sits behind an offering, it remains rajasic.