March left motion-sick investors in mutual funds adrift in a sea of red. U.S. diversified equity funds slipped an average 0.3% last month. They were up 2.5% for the first quarter as a whole. Stock funds were buffeted by several cross-currents: A slowdown in the U.S. economy, brought on in part by a brutal winter and a labor standoff at West Coast ports. A stronger dollar that’s putting the squeeze on exporters and adding to market volatility.