Co pouzivas, OpenGL ? Zkus tam pridat :

graphics.PreferMultiSampling = true

MultiSampleCount = si nastav dle potreby, vyzkousej 8 :

private void Graphics_PreparingDeviceSettings(object sender, PreparingDeviceSettingsEventArgs e)
{
    graphics.PreferMultiSampling = true;
    e.GraphicsDeviceInformation.PresentationParameters.MultiSampleCount = 8;
}