I'd have to see the artwork/bathing suit in question to comment on that, but I am looking at the first "T"-rated hit, right now, which is "Momo and Z Broly SSJ 1" and the character is mostly naked/effectively in underwear - as well as showing some underboob. I think that "T" is the fitting rating here.
On NG, "M" is "almost adult"; not "T". "T" is more like "not for everyone/every age group".
As a shorthand you could ask "Would you show that image to the 6 year old child of a friend?" If the answer is "No.", then it is probably not to be rated "E".
The 'Age Guidelines' can be found here:
https://www.newgrounds.com/wiki/help-information/terms-of-use/art-guidelines
Darkwhisperer666
Someone probably got offended and thought "This ain't good for children"
That's the only thing I could think of.
ItaliaNinjArtist
But it's too absurd! It's a normal bikini, not sexy underwear