If only her owner allowed it, she might be popular (maybe) already.
Considering it's Deep Impact, there's no maybe about it. Deep Impact was arguably the most iconic horse in recent era (or at least, at the time Uma Musume was first announced). Deep was the second horse (After Rudolf) to achieve undefeated Triple Crown wins after all. Notably Deep's child, Contrail, who was the third horse to achieve it, is considerably less popular.