A workaround is to add \rm after \starttext.
Please find MWE below.
\definefontfallback[cal-fallback][name:modern][uppercasescript,lowercasescript]
\starttypescript[mainface]
\definetypeface[mainface][rm][serif][pagella][default]
\definetypeface[mainface][ss][sans] [modern] [default][rscale=1.075]
\definetypeface[mainface][tt][mono] [modern] [default][rscale=1.075]
\definetypeface[mainface][mm][math] [pagella][default][fallbacks=cal-fallback]
\stoptypescript
\definefontfamily[mainface][mm][mainface]
\definefontfamily[mainface][rm][pagella][bf=style:bold]
\usetypescript[mainface]
\setupbodyfont[mainface]
\startbuffer
The quick brown fox jumps over the lazy dog
\stopbuffer
\starttext
% \rm % works
{\rm \getbuffer}\par
{\ss \getbuffer}\par
{\tt \getbuffer}\par
{\bf \getbuffer}\par
{\it \getbuffer}\par
{\sl \getbuffer}\par
{\bi \getbuffer}\par
{\bs \getbuffer}\par
{\sc \getbuffer}
Should be Palatino: $x + y$ $f({\bi x})$
Should be Latin Modern: ${\cal I}$ $\mathcal{I}$ $\mathscr{I}$
\stoptext