Is it usual in anime/manga/japanese to use ブツ as onomatopoeia of mumbling?

Question

In My Hero Academia we see that when Midoriya is mumbling they use ブツ to symbolize it. Is this the rule for anime/manga/japanese or does it only happen in My Hero Academia?

Answer 1

ブツブツ or ぶつぶつ (the same word, just in katakana vs hiragana) is a standard Japanese onomatopoeia for mumbling, muttering, or grumbling to oneself. This list of Japanese onomatopoeia includes ぶつぶつ in it, along with many other onomatopoeia that show up in manga.