I just tried it using that exact same approach on a QPlainTextEdit (which is what CV is) and could see no changes at all even though the code compiled just fine.
Code:
QTextBlockFormat fmt;
fmt.setLineHeight(150, QTextBlockFormat::ProportionalHeight);
QTextCursor tc = textCursor();
tc.clearSelection();
tc.select(QTextCursor::Document);
tc.mergeBlockFormat(fmt);
So something in QPlainTextEdit is preventing the change.