She thinks Eri might be better for Harima than herself, but she acted upon bringing Harima with the one he loved (Tenma). Remember how even at the beginning of the series Harima was on Karasuma's ass for liking curry instead of Tenma. Or when he found out about Tenma liking Karasuma, instead of sabotaging their relationship he went emo about it and then got over it. Or the birthday thing. I think overall he loves Tenma but wants her to be happy with Karasuma. True love doesn't have to be mutual.
Actually, Yakumo seems to think Eri is the one for Harima after Chapter 216, and Harima has spent most of the series trying to sabotage Tenma's relationship with Karasuma. Also, I thought that true love was supposed to be mutual.
Yakumo understands Harima the best out of the three (Tenma and Eri). "A strong heart that won't sway" That seems to perfectly describe him, I don't think that's very naive considering the amount of tunnel vision he has for Tenma.
Yakumo’s opinion of Harima in Chapter 206 sounds rather naive, to me; she reveals an elevated perception of Harima in this internal monologue, characteristic of one a teenage girl would have in a crush. Though, I would hesitate in calling Yakumo’s feelings an example of limerence, as well, since girls have strong romantic feelings towards their desired one in crushes.