Good quality fish oil is a healthy supplement but most effective if you are replacing bad fats with good fats like fish, avocado, a small amount of nuts, and other good oils having low LDL and higher HDL. You would need to be getting enough exercise, eating the right amount of calories, and getting enough fiber. It's here where the fish oil kicks in and starts doing its best work. Walking is great exercise (it's what humans were meant to do much of the time). Fish oil from regular dietary fish consumption is also very good. The omega-3 oils in fish oil, hemp oil, borage oil, and evening primrose oil have been the subject of many studies. Nearly all of them work to decrease inflammation including that of the blood vessels. They seem to effect the immune system and the brain quite positively, and thin the blood by affecting platelets stick together less.
The cholesterol-lowering edge is small but substantial once you have eliminated the bulk of your cholesterol consumption and put yourself in a zone where fish oil can do it's best work. Failing this, it won't hurt to take fish oil- but it might not help as much as you hope until you alter the habits that led to having high cholesterol. With bad habits you'll just keep erasing your progress.
Regardless of your cholesterol levels, there are plenty of good reasons to take omega-3 oils, so you should consider taking them. Many modern food sources are largely devoid of omega-3 oils- but the same animals and plants raised properly are rich in them. The diet of 100-200 years ago was full of omega-3 oils from game meat and homegrown or wild seeds, nuts, and other foods. Recently in human history we learned how to make lots of cheap food- but unfortunately we did not learn how to make lots of cheap healthy food. Fish oil and hemp oil are a good way to replace that lost omega-3 that seems to be part of the diet profile nature (or God if you look at it that way) intended for us humans.