No. Coconut oil does not reduce acidity in coffee. In fact, coconut oil will make your coffee slightly more acidic. This is due to the natural release of a very small percentage of free fatty acids in coconut oil during processing. It's inevitable. Which makes coconut oil a bit acidic by itself – about pH 5.
This is why coconut oil causes scratchy throat when you drink it straight from the jar.
In that sense, you should cut back on adding too much coconut oil to your coffee if you do have acid reflux (heartburn) issue. I suggest starting with one teaspoon of coconut oil added to coffee if you're new to putting coconut oil in coffee.
On top of that, reduce the amount of sugar added to coffee as sugar can easily raise the acidity in coffee, making your combo of coconut oil + coffee even more acidic.