В assistante написано, что шрифт зависит от настроек операционной системы, поэтому имеет смысл установить шрифт самостоятельно
void QApplication::setFont ( const QFont & font, const char * className = 0 ) [static]
Changes the default application font to font. If className is passed, the change applies only to classes that inherit className (as reported by QObject::inherits()).
On application start-up, the default font depends on the window system. It can vary depending on both the window system version and the locale. This function lets you override the default font; but overriding may be a bad idea because, for example, some locales need extra large fonts to support their special characters.
Warning: Do not use this function in conjunction with Qt Style Sheets. The font of an application can be customized using the "font" style sheet property. To set a bold font for all QPushButtons, set the application styleSheet() as "QPushButton { font: bold }"
See also font(), fontMetrics(), and QWidget::setFont().
