I can’t find it either and, get this, when I try to input the url for the search page in the launcher it redirects to the top games section. The app search bar also doesn’t show adult games even if you have the “show adult content” toggle on. Both these facts lead me to believe this is entirely on purpose specifically to stop you from being able to access adult itchio games through the app. You can still download them and see them in your collections but you can’t actively look for them in the app. It’s probably because of the whole PayPal censorship thing going on.
If you’re looking for games just use the browser version and save them, then you can find them in your collections in the app and download them there.