By default, the resolution of the 3D graphics is set to 1080px. If your screen allows a higher resolution, and if your PC runs Girlvania at a good frame rate, you can try to set the "3D Resolution" in-game setting (upper right corner of the screen during game play) to "Max".
This will change the game resolution to match your current desktop resolution, and will make the graphics appear much more clear. However, if your screen has a very high definition, this may impact the performance greatly. If the game becomes unusable, you can revert to a lower "3D Resolution", or you can set a lower "Screen resolution" as described in the previous question.
(Updated: March 10, 2015) You can smooth the jagged edges of the 3D graphics by enabling the "Anti-aliasing" in-game setting.