Can tretinoin be used on top of pustules? Is it helpful?

Yes and no. Tretinoin is a prescription medicine used in acne and typically helps with comedonal acne (blackheads) and typically the topical antibiotics are more helpful for pustules. However if you are having pustular acne you already have comedonal acne so you do need a medication like tretinoin to help open up the pores. Use daily as prescribed.