It's actually not bad. It's just flat, unprocessed and the background color isn't doing much for the shot. The dust spots are distracting and there's a bit of an overall green/olive cast.. probably from the wall color.
Yep, you could go ahead and stop your lens down a bit, since its a solid background.. will give you more DoF and your lens will probably return a sharper result regardless. A slower shutter speed probably wont be an issue either since youre using flash.