Is this considered mask of pregnancy? Ever since I got pregnant it's just been accumulating more and more across my tummy and thighs.

I can't really tell based on the photo if it's melasma, but it could be if it's darkened skin. It usually happens on the face, but sometimes it can happen on the forearms or parts of the body that are exposed to the sun. Melasma is usually splotchy and is triggered by hormonal changes in the body during pregnancy that increases the production of melanin (what gives skin, hair and eyes their color). Don't worry, as it should disappear after pregnancy. In the meantime, you can use sunscreen to minimize the skin darkening.