iMessage apps are not independent apps. They are extensions that are provided by actual apps from the App Store. So, to uninstall one completely you would uninstall the associated app as normal.
If you want to keep the app but remove the visibility of the iMessage extension you can do so by scrolling to the right of the imessage app bar, choosing “More” then swiping to delete any unwanted apps. A word of caution: I can edit, add, and reorder this list on iPad but it crashes when I hit Edit on iPhoen every time. If it’s the same for you you won’t be able to add any apps back so don’t delete anything you may want to keep.