If I create a simple textField:
textField = new TextField(200, 40, ""); addChild(textField);
And then I put my game in Fullscreen and adjust the size of the viewport to so that I use all the available screen space, the textfields become blurry.
But I can fix it by doing :
textField.text = textField.text+" ";
So it's not a problem for me but I thought I should report it.