Conditional bracketing in macro

This checks if + or - is in the argument.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\cat}{om}
 {
  \IfValueT{#1}{ \is_cat_prefix:n { #1 } }
  \mathbf{#2}
 }

\cs_new_protected:Nn \is_cat_prefix:n
 {
  \str_if_in:nnTF { #1 } { + }
   { (#1) } % there is +
   {
    \str_if_in:nnTF { #1 } { - }
     { (#1) } % there is -
     { #1 }
   }
 }
\ExplSyntaxOff


\begin{document}

\(\cat{Bord}\), \(\cat[n]{Fus}\), \(\cat[2]{Rep}\), \(\cat[n+1]{Fus}\), \(\cat[n-k-2]{Rep}\)

\end{document}

I use listofitems here to check for the literal presence of + or - in argument #1 (others can be added).

\documentclass{article}
\usepackage{listofitems}
\newcommand{\cat}[2][\relax]{%
  \setsepchar{+||-}%
  \readlist\checksgns{#1}%
  \ifnum\listlen\checksgns[]>1\relax(#1)\else#1\fi\mathbf{#2}}
\begin{document}
    \(\cat{Bord}\), \(\cat[n]{Fus}\), \(\cat[2]{Rep}\), \(\cat[n+1]{Fus}\), \(\cat[n-k-2]{Rep}\)
\end{document}