I want to have sans serif for the text inside the frame.

While the first example is showing the text with serifs,
the second example is typeset in sans serif indeed, but unfortunately without the first letter (m here).

\starttext

%first example

\framed[frame=on, corner=round, radius=.3em, location=low]
\ss mmk
\stopframed

%second example
{\ss
\framed[frame=on, corner=round, radius=.3em, location=low]
mmk
\stopframed
}

\stoptext