Should I Delete Quarantined Items in Malwarebytes?

Files that have been quarantined are no longer a threat to your computer. It is advisable to leave them alone long enough to ensure your computer is functioning normally. Why? A file that has been improperly identified as malware (a false positive) will be impossible to restore if you have deleted it.

Is it OK to delete quarantined items?

Quarantined files are not deleted unless you want them to be. ... You will need to instruct your antivirus to delete the file or otherwise delete it yourself manually. You can keep a file in quarantine indefinitely, but if an important file that is infected, you should place it in quarantine and clean it.

Is it safe to delete quarantined files in Malwarebytes?

Yes you can. It's probably best to leave it alone though for at least a few days to confirm that you don't encounter any negative impact or that it was not a "False Positive". If you're confident that the file removed was not a False Positive and their is no negative impact you can delete it from quarantine at anytime.