I can't speak to the effectiveness for mango trees, but for various subtropicals that need lower pH soils I have had great luck with tons of coffee grounds. I guess I've had better luck with this than others have, because most folks I've suggested it to haven't found it to work as well, but I think it's because I use way more grounds. I get them from coffee shops and give each tree a lot (during the growing season you can give each tree a 5 gallon bucket of grounds every couple weeks, mixed with a bit of tree trimmings / woody mulch to prevent it from forming a hard layer).
Opinions on this are all over the place. It would be best to do an actual test of the PH of the grounds. Using sulfer is a more surefire way to lower soil PH. Or if you garden on a small scale, using RO water and PH adjusting the water with nutrients/buffers is best.
http://www.gardenmyths.com/coffee-grounds-acidifies-soil/
I do use sulfur, sometimes. But I prefer coffee grounds because: 1) it builds soil organic matter, 2) worms seem to love it, 3) it serves as a mild, relatively balanced fertilizer, 4) it gently adjusts pH, 5) it's widely available, and 6) most importantly, it seems to work really well to correct the problems associated with high pH soils for subtropicals around here.