I do not really know what simplify means because this is already simple IMHO. You can do something like this:
```
\def\searchlist{/tikz,/pgf,/bee,/marmot}
\tikzset{%
/thing/.search also/.expanded={\searchlist},% <- simplify this line
/thing/.cd,
hat/.code = \thinghattrue,
}
```
But I do not know what the aim is. Full code.
```
\documentclass{standalone}
\usepackage{tikz}
\def\tikzlinglist{
bee/bees,
marmot/marmots%
}
\def\otherlist{
bee,
marmot%
}
% Bee %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\tikzset{
/bee/.search also={/tikz,/pgf,/thing},
/bee/.cd,
body/.code = \def\beebody{#1},
body = yellow!70!orange,
}
\newcommand*{\bee}[1][]{%
\begin{scope}%
\tikzset{/bee/.cd,#1}%
\fill[\beebody] (0,0) circle [radius=1];
\end{scope}%
\thing[#1]%
}
% Marmot %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newif\ifmarmotblush
\tikzset{
/marmot/.search also={/tikz,/pgf,/thing},
/marmot/.cd,
body/.code = \def\marmotbody{#1},
body = brown,
blush/.code = \marmotblushtrue,
}
\newcommand*{\marmot}[1][]{%
\begin{scope}%
\tikzset{/marmot/.cd,#1}%
\fill[\marmotbody] (0,0) circle [radius=1];
\ifmarmotblush
\fill[red] (-0.5,-0.2) circle [radius=0.1] (0.5,-0.2) circle [radius=0.1];;
\fi
\end{scope}%
\thing[#1]%
}
% Addons %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newif\ifthinghat
\def\searchlist{/tikz,/pgf,/bee,/marmot}
\tikzset{%
/thing/.search also/.expanded={\searchlist},% <- simplify this line
/thing/.cd,
hat/.code = \thinghattrue,
}
\newcommand*{\thing}[1][]{%
\begin{scope}%
\tikzset{/thing/.cd,#1}%
\ifthinghat
\fill[black] (0,1) ellipse [x radius=1,y radius=0.4];
\fi
\end{scope}%
}
\begin{document}
\begin{tikzpicture}
\bee[body=yellow]
\marmot[blush,xshift=2cm,hat]
\end{tikzpicture}
\end{document}
```