No it is a myth. Sure there is a study that shows that drinking acai actually increases your calories burnt by a few calories...but you take in more calories from a concentrated acai drink than you burn. taking acai pills is a similar sort of deal yes you will burn literally 1 or 2 extra calories...but you'd do more by eating a tea spoon less potato, or rice, the difference is that small.
The best way to lose weight is to control portion size. A simple way is to change the calories you eat is to have slightly less of your carb source (like rice potato quinoa etc) & to increase your green leafy veggies. That way you have the same volume of food, but less overall calories (& more nutrients from the green veggies!).
food is the best way to control diet. fat burners don't work on the average person. Sure if you are a contest bodybuilder & you're going from 6 to 5% bodyfat then they might swing it for you, but for the average person the amount of extra fat burnt is so low that even the tiniest increase in food (like a teaspoon or so) more than overcompensates for any fat burning effect.
Think of changing your lifestyle, not dieting. Dieting is something you go on, then come off, usually it doesn't work. Lifestyle changes mean changing the way you live from now on. It will include more sensible dietary choices (not giving up all the foods you like, but maybe limiting them to one or 2 meals a week), also including some weight training & cardio work (depending on your timetable this can be as little as 3 times a week to as much as 6 times a week). Also trying to walk more, drive less.
There are many ways to change your body composition. Let us know where you are now, what your goals are & what you think you are doing right & wrong & we can try & come up with some options you might want to incorporate into your life that will make you leaner & in better condition without the need of quick fixes like acai or other stuff that really does nothing for the average person looking to lose fat.