Yes. Flashback is just an alternative cost (CR 702.34a):
Casting a spell using its flashback ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.
and 601.2f explicitly allows reductions to apply to alternative costs:
The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions.
(Rule references to the 2023-02-03 Comprehensive Rules)