Is there a modal command for MakeLowercase?

Really there is no such command.

If you have a command that acts as a switch, such as a font change like \bfseries then it is easy to make a command that takes an argument and applies the original command in a local group, so \textbf is (simplified a bit) just

\def\textbf#1{{\bfseries #1}}

However going the other way is harder. \MakeUppercase is a wrapper around \edef and \uppercase and both of these primitives require a {} delimited list of tokens on which to act.

You can get caps and small caps as a modal switch \scshape as that font shape is commonly available, however an all-caps font is not available in most font sets. \uppercase (and so \MakeUppercase) does not work as a font change but as a token level transformation replacing each token in the argument by a specified replacement a to A for example. Conversely with an all caps font then a would still be a (ASCII/Unicode hex 61) but would use a glyph that looked like A when rendered.

You can define such control sequence as macro, if you need it. The following code defines \lowc.

\def\lowc{\expandafter\lowercaseA\expandafter{\iffalse}\fi}
\long\def\lowercaseA#1{\lowercase{#1}\egroup}

Test: {\bf TEXT \lowc AHA UFF}

\bye