You should! Or at least some bark or other ground cover. Looks a lot better. I assume that you're going to make holes in the plastic for planting? The weeds will find a way through those holes. You should put something down after you've planted so it looks nice. Have fun!
No, the only reason to do that would be to keep plastic from blowing off. You can remedy that with a few bricks or u-shaped anchors to pin plastic down. If you put dirt on top you will have weeds on top of your plastic.
Picture a sheet of plastic with flowers coming up through it. Not very attractive is it? Aerate your soil good, churn it up, use a good weed killer throughout. Good fresh soil and nutrients and mulch. Plant the flowers you like that are appropriate for your zone and sun/ shade conditions. I have used plastic before but as you will find, the weeds do come up, and the bugs do crawl through. Sooner or later you'll have messy pieces of plastic strips through your flowers. I use bark atop the soil after I plant. It looks pretty and helps keep moisture in. When I see a weed, I yank it ;)