mgarin/weblaf

Yep, exactly the same code I've used in my test class:

public class FontTest
{
    public static void main ( final String[] args )
    {
        SwingUtilities.invokeLater ( new Runnable ()
        {
            @Override
            public void run ()
            {
                try
                {
                    UIManager.setLookAndFeel ( WebLookAndFeel.class.getCanonicalName () );
                    // UIManager.setLookAndFeel ( MetalLookAndFeel.class.getCanonicalName () );

                    final Font font = Font.createFont (
                            Font.TRUETYPE_FONT,
                            FontTest.class.getResourceAsStream ( "Lato-Regular.ttf" )
                    ).deriveFont ( 16f );

                    final JFrame frame = new JFrame ();
                    frame.setLayout ( new FlowLayout () );

                    final JTextField textField = new JTextField ( 50 );
                    textField.setFont ( font );
                    frame.add ( textField );

                    frame.setSize ( 800, 600 );
                    frame.setLocationRelativeTo ( null );
                    frame.setDefaultCloseOperation ( WindowConstants.EXIT_ON_CLOSE );
                    frame.setVisible ( true );
                }
                catch ( final Exception e )
                {
                    e.printStackTrace ();
                }
            }
        } );
    }
}

I recommend you to try this small example and see if the same issue occurs or not, once with WebLaF and once with the MetalLookAndFeel.

If it doesn't appear on this example - it might have something to do with other settings in your application.
If it does - then it could be a Java issue on Mac OS, especially if it appears for you on both L&Fs.