While KMail isn’t necessary at all to use KDE (even Neon uses Thunderbird as its default email app) I think I know what you’re getting at. The issue isn’t KMail itself. Installing KMail will also install Kontact, which runs with Akonadi server on the back end, and Akonadi is the culprit for it being so “far embedded” into Plasma and on some setups can use up just as much RAM as Plasma itself on its own. It’s my main complaint about KMail/Kontact/Akonadi, if they could get the RAM usage under control (you seriously don’t need to use almost a freaking GB of RAM just to keep track of email, contacts and a calendar, Thunderbird does it using less than half the memory), it would be a really nice system for one bundled with a DE.