Thank you for your answer.
If I understand you correctly, deleting this account will remove/delete the Jetpack Plugin, (which I have through this account) but will leave the rest connected through Siteground?
It won’t remove the plugin, but it will disconnect the plugin from WordPress.com, meaning any features added to your site by the plugin like stats, WordPress.com themes, some widgets, etc, will stop working. But deleting the WordPress.com account won’t otherwise affect your Siteground site in any way – it will still be online, connected to your domain, and accessible to anyone based on your settings over there.
To remove the Jetpack plugin completely you’ll need to log into WP-Admin on your site and delete it from there.