Sorry, but \medskip
does not do what you expect to.
Consider the simple example
\documentclass{article}
\begin{document}
\begin{tabular}{cc}
\hline
a & b \\
\hline
\medskip
c & d \\
\hline
\end{tabular}\qquad
\begin{tabular}{cc}
\hline
a & b \\
\hline
c & d \\
\hline
\end{tabular}
\end{document}
and its output

As you see, the vertical space is added below the second row, not between the first and second row.
You want, instead, to use siunitx
and booktabs
:
\documentclass{article}
\usepackage{siunitx,booktabs,array}
\begin{document}
\begin{tabular}{
>{\bfseries}l
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2e-1]
S[table-format=1.2e-1]
}
\toprule
\multicolumn{1}{c}{\textbf{Model}} & \multicolumn{5}{c}{\textbf{Parameters}}\\
\cmidrule(r){1-1} \cmidrule(l){2-6}
& $\mu$ & $\alpha$ & $\beta$ & $\lambda_p$ & $\lambda_b$ \\
\midrule
Poisson & {--} & {--} & {--} & 2,66e-2 & {--} \\
\midrule
Hawkes & {close to 0} & 3.53 & 3.97 & {--} & {--} \\
\midrule
Branching & {--} & {--} & {--} & {--} & 0,13e-2 \\
\bottomrule
\end{tabular}
\bigskip
\begin{tabular}{
>{\bfseries}l
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2e-1]
S[table-format=1.2e-1]
}
\toprule
\multicolumn{1}{c}{\textbf{Model}} & \multicolumn{5}{c}{\textbf{Parameters}}\\
\cmidrule(r){1-1} \cmidrule(l){2-6}
& $\mu$ & $\alpha$ & $\beta$ & $\lambda_p$ & $\lambda_b$ \\
\midrule
Poisson & {--} & {--} & {--} & 2,66e-2 & {--} \\
Hawkes & {close to 0} & 3.53 & 3.97 & {--} & {--} \\
Branching & {--} & {--} & {--} & {--} & 0,13e-2 \\
\bottomrule
\end{tabular}
\end{document}
I'd prefer to avoid as many horizontal rules as I can, so I showed also the table without the unnecessary ones.

The second table, with some added vertical space and no boldface:
\begin{tabular}{
l
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2]
S[table-format=1.2e-1]
S[table-format=1.2e-1]
}
\toprule
\multicolumn{1}{c}{Model} & \multicolumn{5}{c}{Parameters}\\
\cmidrule(r){1-1} \cmidrule(l){2-6}
& $\mu$ & $\alpha$ & $\beta$ & $\lambda_p$ & $\lambda_b$ \\
\midrule
Poisson & {--} & {--} & {--} & 2,66e-2 & {--} \\
\addlinespace
Hawkes & {close to 0} & 3.53 & 3.97 & {--} & {--} \\
\addlinespace
Branching & {--} & {--} & {--} & {--} & 0,13e-2 \\
\bottomrule
\end{tabular}

Removing boldface is good, the additional vertical space is not really important.