In this particular situation, Qt Creator is launched with the main menu properly sized, but the 'internal' elements of the UI too small. See e.g.
One can work around this by manually setting the scale factor + Qt font DPI, e.g.
QT_FONT_DPI=92 QT_SCALE_FACTOR=2 path/to/qtcreator
but it would be nice if Qt Creator could automatically detect if Ubuntu has UI scaling in effect, and adjust these settings to match.