When you take a roller coaster ride there are usually big signs posted everywhere: DON'T USE IF YOU ARE PREGNANT! Only you can decide whether you want to ignore this warning or not.
Unless you are a high risk pregnancy then a roller coaster ride in and by itself will probably not endanger the pregnancy, and there are probably no increased risks of miscarrying when using a roller coaster. But a rollercoaster ride is more dangerous then, let's say, a leisurely stroll on the beach. And miscarriages are not unusual. So if you have a miscarriage after a rollercoaster ride, will you blame yourself for having taking the ride? Do you think it's worth the risk?