That's not really accurate. Here's my very vague understanding:KDE and LXDE what they call "plasma" screens, like what is used in Linux Mint,
KDE calls their desktop interface/environment Plasma. This includes their widgets. In KDE4 everything was glass-like and glossy. This look is toned down a little in Plasma 5, but there is still lots of translucency (like frosted glass) of windows and surfaces.
So "Plasma" is just the name of the environment.
But if you look at the underlying packages and programs that the different DEs build their environments on:
1. KDE uses QT-based technologies (NB. Gnome 3 uses gtk3)
2. As far as I know, LXDE is mainly gtk2 with built in compatibility for gtk3 apps. However, in the wings there is the eventual successor of LXDE called LXQT that is based on QT. Some QT bits might be familiar to those who use KDE, I guess, but overall the 2 DE are quite different.
3. Mint's 2 main DE are Cinnamon, which is a fork of Gnome 3 (so it's more gtk3), and MATE which is gtk2 with gtk3 support. However, there is also a Mint KDE edition, so you can get your Plasma there. :-)