It's just part of the language. Everything has onomatopoeia. It's not just Toriyama who does it. You will find it in manga all over even stuff before his time. You'll also hear things like "jiiiiii" for people staring in anime too.
Uh, I know. I am Japanese, I have tons of manga reading experience over decades. It's just that reading the same work both in the original Japanese and translated English made the difference in threshold more obvious.