Is it safe to uninstall SIMBL agent on Yosemite?
The easiest way to uninstall SIMBL is to run the its bundled uninstaller.
You can get a copy from the official website.

After the uninstallation check if these folders are present:


~/Library/Application Support/SIMBL
/Library/Application Support/SIMBL


If they are present you can easily delete them.

Since OS X 10.11+ SIMBL gets broken due to the introduction of SIP by Apple (System Integrity Protection).

There is no need to disable SIP before the process and re-enable it later.