While Izuku is the protagonist, "That’s the idea" applies equally to Ochaco’s growth. Initially, her motivation is purely financial—to support her parents. While noble, she often felt her "practical" goals were less heroic than the grand ideals of her classmates.
The most immediate significance of this idea is the reclamation of the name "Deku." Originally a slur meant to denote helplessness (meaning "one who can't do anything"), Ochaco flips the script by suggesting it sounds like dekiru ("you can do it"). That’s the Idea, Ochaco
Ochaco’s ultimate realization is that a hero’s job isn't just to punch villains, but to see the humanity in the people society has discarded. When she stands on the roof of U.A. to defend Izuku to a fearful public, she is executing the ultimate "idea": that heroes and civilians must protect each other. Conclusion While Izuku is the protagonist, "That’s the idea"