No personal data is collected by the extension. The extension just scans the page for any known diagrams, so he code block of the found diagrams is encoded and sent to a server to obtain its graphic form. No other data is used or sent - all the code is open source and inspectable.
By default the extension sends diagrams to the free public cloud instance https://kroki.io; normally this is not a security issue, but you can still install Kroki on your own personal infrastructure (and change extension config). In the extension settings you can however set a deny-list of sites, in which the extension will be kept deactivated.