Monofilament wigs can be flatter than their permateased sister styles, but there are some things you can do.
There is a pretty easy way to get a little more volume without resorting to permanently altering the style with steam, though!
You can always try washing it and drying it upside down. That will usually put some temporary lift in. Some synthetic safe styling spray might also help, especially if you store the wig upside-down (like in your closet) and spray it while it's off your head.
One product I recommend is the Wigmate Dry & Style. It is GREAT for this! Use it with Jon Renau's Wig Styling Spray and you should be able to get some more body out of your monofilament top without damaging it. :)