The % range for truly killing most characters is very narrow (like 5% or less). It's also different for every character. The only characters who have a relatively wide % range are very floaty characters like Rosa, Peach and Samus. Having light weight is also a factor.
Things which make it easier to kill the opponent include:
- Starting with a full hop Uair rather than a short hop
- Not doing fastfall Uairs during the combo
- Starting the combo from a platform
- Getting the sweetspot of Shuttle Loop (the lower hitbox)
- Comboing them off the side and finishing with Nair, Bair, airdodge read > Drill etc.
MK has lots of other kill options so it's not totally necessary for Uair combos to kill, racking up like 40% is amazing in of itself. It is good to take a stock when you get the opportunity though of course.