what difference does it make? it's just a text editor, most of them are few MB big, I don't expect QT apps to be more lightweight than the apps out there, that being said, and since most Linux distro have GTK in them, I don't understand why a QT notepad would even matter.
I understand some users not wanting to install KDE apps on gnome desktop because it will install many dependencies. But wanting QT apps just because they are QT apps, I don't understand that, certainly not to something as simple as notepad.
Having said that, take a look at TEA editor, they say that they offer two versions, a GTK version and a QT version, I quote
TEA is a text editor that provides a wide range of text-processing
functions (over 100) and the syntax highlighting. There are two
branches of TEA: Qt-based and GTK-based.
That's all I know about it, you can find more on the about page. I never tried it and not interested in doing so. However the project seems inactive, their last release was in 2013. But it looks like a mature project, and it is okay if it's inactive, it's just a text editor, no rocket science, no need to update it so often.
Very few project match Notepad++ or even better, Gedit, sublime text, vim and emacs. Maybe texmate. All the other projects are not even close and no developers interested in reinventing the wheel unless there's a need to, something i don't see. And there's no money in making text editors. Few donations at best.