When opening qtquickcontrols2.conf in editor, Qt Creator shows warning/pop up toast about syntax highlighting:

A highlighting definition was not found for this file. Would you like to download additional highlight definitions?

Clicking on "Download definitions" indeed downloaded something about Swift, Julia and others, but in the end it did nothing for the *.conf file in question.

In my opinion, Qt Creator should be smart enough to integrate with QML concepts, and treat all qtquickcontrols2.conf files as ini. It should be no harder than adding a filename to a list of recognizable patterns of a syntax definition.