though this is extremely old, i have been struggling with this myself, so maybe it will help somebody else searching
in the future to "close" the issue:
there seem to be two issues:
1. mrxvt does not (yet) support utf8. if using a utf8 locale, mrxvt will "choke" on it, especially with ncurses apps.
if you can live without it within mrxvt, using a "standard" ascii locale is a workaround (works for me: mutt looks
nice and smooth again). documented here:
2. the rxvt term info definitions seem to be somehow problematic, the newer mrxvt uses TERM=rxvt, thus there are
further issues within ncurses. using -tn xterm (or equivalent config option for mrxvt) works around this (ex.: it fixes
some drawing issues in aptitude).
as a side note, i use fvwm, which now supports tabs natively, so i shouldn't need the main mrxvt feature which was a
"deal maker" for me back when i chose it, but i am too lazy for now to configure the fvwmtab module
hope this helps somebody.