Remember when Yoda said “Wars not make one great” in Empire Strikes Back? He learned that from the Clone Wars.
It’s only through the teachings of Qui-Gon, who became his master after Revenge of the Sith and was basically the perfect Jedi, even if nobody could see it, was Yoda able to grow into the character he is in Empire Strikes Back.
I’d find this a lot more compelling if this was an actual plot thread in the prequels, instead of something people just inferred from a bunch of EU media.
When Yoda says this in Empire, it’s not hard to imagine he’s saying this from firsthand experience. But in the prequels, that’s really not what’s portrayed. Yoda has no character arc where he learns the horrors of war. If it’s true that he learned this through Qui-Gon, you gotta remember that this is entirely off-screen, after the credits roll in Episode 3.
It is a plot thread in the prequels. It is clear by the end of ROTS that the war was a mistake, given that Palpatine engineered the whole thing for his own ends, and it lead to the near extinction of the Jedi order. Yoda obviously recognizes this and learns from it as his open admission that he’s failed and his exiling himself to Dagobah bears out.