Really since mojoPortal is always evolving, it always best to use the very latest version of Mono. I know this is difficult on Debian as they are always behind in packages. Mono is always getting closer to the .NET framework but the framework keeps evolving too with ajax stuff and Mono has been having to keep up with new things too and they are always a little behind .NET in terms of supporting all the features of .NET.
I am not sure the current build of mojoportal can run on 1.9.1 Mono but even if it can I'm not sure whether it had support for CSS Control adapters, without which the menus in mojoportal skins don't render as they should. I think its only as of Mono 2.0 that this was supported.
Its not that difficult to build Mono from svn and to me that is the best strategy as it keeps you up on the very latest supported stuff and the latest bug fixes. Occasionally bugs and regressions get introduced but if you keep a log of each revision you can always revert to the last one that worked well.