Bro, 100 calories for a whole coconut. The water is 66 calories for 300ml and the isi adds 34 calories, so we are talking about a legit low-calorie option that actually fills you up a bit. The 21g of carbs here is a mix of natural sugars from the water and flesh, and at that total it is not going to wreck your deficit if you are eating one a day. This is the drink you want to be ordering at the pasar malam instead of that teh tarik with three teaspoons of condensed milk.
The thing that changes the maths here is whether you eat the flesh or skip it. Air sahaja, just the water, is only 65 calories. Add the isi kelapa muda and you get 34 extra calories for 90g of soft jelly, which is honestly worth it because the flesh keeps you more satisfied. What you want to avoid is the blended versions some stalls sell with added sugar or syrup, because suddenly your 100-calorie coconut turns into a 200-calorie dessert drink. Stick to the plain, chop it open in front of you version and you are fine lah.