It happens to be the correct term for words that represent noises, in English anyway.
Technically many examples of Japanese onomatopoeia aren't actually onomatopoeia because they don't represent sounds that actually occur, such as jiii for staring.
However, I don't think we want to get into trying to get people to figure out the difference between giseigo, giongo, gitaigo, and gijougo for tagging purposes.
Personally, I don't care for a general noise tag at all. It's practically a mandatory tag for almost any Japanese work that has sounds in it at all -- and many that don't thanks to gitaigo and gijougo, e.g. pool #1534.
P.S. We do have some poorly used tags for a few specific sounds. Of the top of my head, the only one I can think of though is kupaa.