\documentclass{article}

\usepackage{mathtools}

\makeatletter
\newcommand{\@distr}[2]{\underbracket[0.3pt][1pt]{#1#2}}
\newcommand{\distr}[1]{\mathpalette{\@distr}{#1}}
\let\oldmathcal\mathcal
\renewcommand{\mathcal}[1]{%
  \oldmathcal{#1}
  \@ifnextchar\distr\!\relax
}
\makeatother

\begin{document}

\[
  a_f\ a_{\distr{f}} \qquad \mathcal{L}\psi\ \mathcal{L}\distr{\psi}
\]

\end{document}