Cleveref and pdf Bookmark

I'm afraid there is no solution for this, apart from using the command \texorpdfstring, that is you have to specify what has to go in the text and what has to go in the bookmarks.

Hence, substitute

\section{Proof to \Cref{some label}}

with

\section{Proof to \texorpdfstring{\Cref{some_label}}{Theorem \ref{some_label}}}

Note that it isn't a good idea to have label names with spaces, so I've substituted some label with some_label.

MWE:

\documentclass{article}

\usepackage{hyperref}
\usepackage{cleveref}

\newtheorem{theorem}{Theorem}

\begin{document}

\section{Section 1}
\begin{theorem}\label{some_label}
    This is a theorem.
\end{theorem}

\section{Proof to \texorpdfstring{\Cref{some_label}}{Theorem \ref{some_label}}}

\end{document} 

Output:

The \Cref command cannot be used in \section when bookmarks are being produced; however, \autoref works (it's less powerful, though). For a simple application like this you can do as follows:

\documentclass{article}

\usepackage{hyperref}
\usepackage{cleveref}

\pdfstringdefDisableCommands{\let\Cref\autoref}

\newtheorem{theorem}{Theorem}

\begin{document}

\section{Section 1}

\begin{theorem}\label{some_label}
This is a theorem.
\end{theorem}

\section{Proof to \Cref{some_label}}

\end{document}