Align position of decimal point within table of numbers, text and percentage values

It's probably better to use siunitx; the \sisetup settings will be local to this table, since the command appears in the table environment.

\documentclass[a4paper,12pt]{scrreprt}
\usepackage{booktabs}
\usepackage{siunitx}

\begin{document}

\begin{table}[htpb]

\sisetup{
  table-space-text-post=\%,    % leave space for a ‘%’
  table-align-text-post=false, % push ‘%’ next to the number
}

\centering

\begin{tabular}{
 l
 S[table-format=6.4] % six integer digits, four decimal ones
 S[table-format=4.0]
 S[table-format=3.4]
 S[table-format=3.3]
}
\toprule
\multicolumn{1}{l}{Test 1} & \multicolumn{2}{c}{Test 2} & \multicolumn{2}{c}{Test 3} \\
\cmidrule{1-1}\cmidrule(lr){2-3}\cmidrule(l){4-5}
Observations successful &
\multicolumn{1}{c}{no} & 
\multicolumn{1}{c}{yes}  &
\multicolumn{1}{c}{maybe} &
\multicolumn{1}{c}{no}  \\
\midrule
adsadad & 546546.546 & 4646 & 45.646 & 456.456 \\
\% AAA & 235.25\% & 22\% & 232.34\% & 234.25\% \\
\% CCC3 & 0.0033 & 0 & 0.0031 & 23 \\
\% CCC4 & 0.0033 & 0\% & 0.0031 & 0 \\
\bottomrule

\end{tabular}
\end{table}

\end{document}

\documentclass[a4paper, 12pt]{scrreprt}
\usepackage{setspace}
\usepackage{booktabs}
\usepackage{floatrow}
\floatsetup[table]{capposition=top}
\usepackage{dcolumn}
\newcolumntype{Y}{D..{6.4}}
\newcommand\pct{\rlap{$\%$}}
\begin{document}

\begin{table}[htpb]
\begin{tabular}{lYYYY}
\toprule
\multicolumn{1}{l}{Test 1} & \multicolumn{2}{c}{Test 2} & \multicolumn{2}{c}{Test 3} \\
Observations successful &
\multicolumn{1}{c}{no} & 
\multicolumn{1}{c}{yes}  &
\multicolumn{1}{c}{maybe} &
\multicolumn{1}{c}{no}  \\
adsadad & 546546.546 & 4646 & 45.646 & 456.456 \\
AAA & 235.25\pct & 22\pct & 232.34\pct & 234.25\pct \\
CCC3 & 0.0033 & 0 & 0.0031 & 23 \\
CCC4 & 0.0033 & 0\pct & 0.0031 & 0 \\
\bottomrule

\end{tabular}
\end{table}

\end{document}