I use MPQ Editor to extract the appearance files which works just fine.
When I try to convert those .app files to .obj using D3AppConvert2Obj.exe I get errors. I've tried it on numerous .app files.
System.ArgumentException: The output char buffer is too small to contain the decoded characters, encoding 'Unicode (UTF-8)' fallback 'System.Text.DecoderReplacementFallback'.
Parameter name: chars
Blizzard repackaged a lot of their files with the release of Reaper of Souls so the older ones don't work anymore. Try using Taylor Mouses. It worked the last time I used it. Plus it is better than an OBJ convertor.