\documentclass[a4paper]{scrartcl}
\usepackage[utf8]{inputenc} % this is needed for umlauts
\usepackage[ngerman]{babel} % this is needed for umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf

\usepackage[locale=DE]{siunitx} % Formats the units and values
\usepackage{datatool}
\usepackage{booktabs} % For \toprule, \midrule and \bottomrule
% start every dtl table with \toprule from booktabs
\renewcommand{\dtldisplaystarttab}{\toprule}

% likewise for \midrule and \bottomrule from booktabs 
\renewcommand{\dtldisplayafterhead}{\midrule}
\renewcommand{\dtldisplayendtab}{\\\bottomrule}

% Setup siunitx:
\sisetup{
  round-mode          = places, % Rounds numbers
  round-precision     = 2, % to 2 places
}

\begin{document}
    \DTLloaddb{stores}{messergebnisse.csv} % adjust the name
    \DTLdisplaydb{stores}
\end{document}