I'm not sure what the uninstaller does as i'm not near my wii at the moment and the manual doesn't give much away at the moment.
It maybe trying to download files from the Nintendo update service so that would need wifi enabled.
Other methods you could use:
Accept Nintendos update, you can do this via the wii settings. Not the best way to uninstall but as long as you've got HBC and ios 249 installed before you do the update you can downgrade if you need to in future.
Another method is to use Waninkokos IOS Downgrader (not the CIOS Downgrader), this will download and install IOS's from Nintendos Update Service (these still have the trucha bug which is useful), you can then use Any Region Changer or the System Menu wad to install System Menu 3.2.
You may also want to run the MiiWii Preinstall if you installed Preloader, this will remove the hacks.ini file, do this after you've done the uninstall