Overall ok, but not impressed. There are many things missing or wrong with the word monitoring and how it is setup.

0 0 * * *, by doing so you are only monitoring it once a day, and can't really call it monitoring, because what happens when you are over the diskspace at 01:00?

Should be doing the monitoring every xx mins (at least more frequent than once a day).

Secondly, it is just a bash script, it would have been nice to provide a post command, which can execute such as clean up logs or rm whatever files that need to be deleted and not already rotated. Which could trigger when a certain threshold is reached.