Yes, but it may not be that much flexible and quick to develop in. There is nothing JS can do that you can't do in C++, but due to its static nature, C++ is a tad less suited for certain tasks.
With QML in particular, JS saves you a lot of time, and you can always go back and convert performance critical parts of your app to C++ code. JS itself is not all that different from C++, well, in a lot of ways it is a lot different, it is dynamically typed, it is interpreted and it has a totally different OOP paradigm, but in terms of syntax it is still a C based language, just as C++.
JS is easy, once you overcome the idiotically designed OOP in JS you will become comfortable with it quickly. Especially for QML, where you will hardly ever need to write OOP JS at all, you might as well ignore OOP and use JS as a simple scripting language, as it was originally intended before someone got the moronic idea to force a scripting language to become an application development language.