No, don't fabric paint an entire dress. It will be stiff like crinkled cardboard. Even watering down acrylic-based fabric paints to spread thinner doesn't stop them from drying a little stiff. It's less noticeable with bits of it all over, say in an all-over pattern painted on, but trying to solidly paint any garment is just not going to be pretty.
Being as it has some poly content, even attempting to strip it with bleach is probably not going to work. Search up some of the threads on dyeing and bleaching to be sure.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com