Yes, it's in the stdlib, but it's too old - version 3.0. Current IUP is 3.17.
I started to learn IUP and here is some helpers that could be useful for you: https://github.com/vegansk/nimtests/blob/master/stdlib/iup/iuputils.nim. Usage examples in the tutorial subfolder. And maybe sometime it will be a library :-)
As vega stated, IUP is still in the standard library but only supports version 3.0. There was an effort to remove IUP from the standard library and make it a nimble package https://github.com/nim-lang/iup. Currently that repo is simply a clone of the iup module from the standard library.
I had made some progress previously to upgrade the library to IUP v3.16 (https://github.com/lazypenguin/iup/tree/3.16.0). My 3.16 branch worked last I used it but there was a few IUP controls that I didn't need and didn't port (e.g. webbrowser). I was planning to submit another pull request once I had that last remaining pieces implemented but I lost steam.