Yes indeed, you can use Monistat, which contains miconazole, while breastfeeding. This medication isn't likely to negatively impact your breastfeeding baby because it's poorly absorbed from the skin and vagina and has poor oral bioavailability. So it's generally safe for use.
However, it's worth noting that when it comes to treating sore nipples during breastfeeding, miconazole ointment seems to have no advantage over lanolin, according to some research. In fact, a survey of the Academy of Breastfeeding Medicine 1 found that topical miconazole is rarely prescribed to nursing mothers to treat thrush.
If you do use it, make sure to remove any excess cream or ointment from your nipples before nursing your baby. And a little tip: opt for water-miscible cream or gel products when applying to the breast, as ointments could expose your baby to high levels of mineral paraffins through licking. Always remember, it's best to consult with a healthcare professional before starting any new medication while breastfeeding.