Actually, the original C++ code has the same problem. You get a better result if you make the angle negative at line 44 of your Python version.

The solution can e simplified a little by getting Qt to do more of the mathematics:
Qt Code: 
  1. #include <QtGui>
  2. #include <cmath>
  3.  
  4. class Widget : public QWidget
  5. {
  6. public:
  7.     Widget ()
  8.         : QWidget() { }
  9. private:
  10.     void paintEvent ( QPaintEvent *)
  11.     {
  12.         QString hw("hello world");
  13.         int drawWidth = width() / 100;
  14.         QPainter painter(this);
  15.         QPen pen = painter.pen();
  16.         pen.setWidth(drawWidth);
  17.         pen.setColor(Qt::darkGreen);
  18.         painter.setPen(pen);
  19.  
  20.         QPainterPath path(QPointF(0.0, 0.0));
  21.  
  22.         QPointF c1(width()*0.2,height()*0.8);
  23.         QPointF c2(width()*0.8,height()*0.2);
  24.  
  25.         path.cubicTo(c1,c2,QPointF(width(),height()));
  26.  
  27.         //draw the bezier curve
  28.         painter.drawPath(path);
  29.  
  30.         //Make the painter ready to draw chars
  31.         QFont font = painter.font();
  32.         font.setPixelSize(drawWidth*2);
  33.         painter.setFont(font);
  34.         pen.setColor(Qt::red);
  35.         painter.setPen(pen);
  36.  
  37.         qreal percentIncrease = (qreal) 1/(hw.size()+1);
  38.         qreal percent = 0;
  39.  
  40.         for ( int i = 0; i < hw.size(); i++ ) {
  41.             percent += percentIncrease;
  42.  
  43.             QPointF point = path.pointAtPercent(percent);
  44.             qreal angle = path.angleAtPercent(percent);   // Clockwise is negative
  45.  
  46.             painter.save();
  47.             // Move the virtual origin to the point on the curve
  48.             painter.translate(point);
  49.             // Rotate to match the angle of the curve
  50.             // Clockwise is positive so we negate the angle from above
  51.             painter.rotate(-angle); 
  52.             // Draw a line width above the origin to move the text above the line
  53.             // and let Qt do the transformations 
  54.             painter.drawText(QPoint(0, -pen.width()),QString(hw[i]));
  55.             painter.restore();
  56.         }
  57.     }
  58.  
  59. };
  60.  
  61. int main(int argc, char **argv)
  62. {
  63.     QApplication app(argc, argv);
  64.     Widget widget;
  65.     widget.show();
  66.     return app.exec();
  67. }
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