It depends on the sport. I grew up playing tennis and basketball (yes I'm left handed) and there was a distinct advantage to being left handed. For one the spin of the ball is different and secondly your opponents forget which side is your strong side. Competitors get used to playing a certain way and it is difficult for them to adjust to a different style. Baseball is by far the most in depth sport when it comes to right handed vs left handed players. Pitching substitutions, pinch hitters, and fielders are all determined based on match ups and scenarios. We always hear about bringing in this guy to face the lefty or pinch hitting this guy because he hits righties well. It's obvious the right hand/left hand dynamic plays a major role in the strategy of the game. On the other hand there is no advantage to being left handed in swimming, track and field, rowing, cycling, hockey, or soccer. Those are sports that require a total body effort as opposed to mostly arms and hands.
I wouldn't go so far as to say left handed people are better athletes than right handed people, but the unorthodox style and the infrequency with which you compete against left handers makes it a challenge to adjust. Even left handed athletes don't want to compete against other left handers. It's confusing when one of your strengths is used against you and becomes a disadvantage. The pendulum swings both ways. Ultimately talent prevails, but being left handed gives you a split second advantage by being different from the norm.