Levers these days are meant for single finger gripping. Most likely this should be your index finger, unless you've got a disabled or removed finger. With most modern mountain bike brakes being hydraulic disc systems, there is no need to apply a ton of pressure to make your bike stop. Obviously if you're riding a set of V-brakes covered in rust, you may need to add some squeeze power with a second finger - but I'm assuming we've all moved past that.
Using only your middle finger will either crush your pointer finger or force you to adjust your levers out dangerously far from your bars. You'll end up squeezing your brakes with the last knuckle of your middle finger and your brakes will be fully applied inch(es) from the bar. This will not only fatigue your hand and forearm quickly, but make you more likely to "panic squeeze" in sticky situations.
Using your index but adding your middle finger onto the lever forces your index finger closer to the base of the lever, making it harder to modulate or "feather" your brakes effectively. In most instances you may only need to apply 15-25% of your braking power. Two fingers make it hard to gently brake. Excessive braking will cause skidding, which equal out of control riding, poor turning, tire bill$, and destruction of our beloved trails. Two fingers is too smashy, you want finesse. One finger.
For a while, everyone was cutting their bars more and more narrow. We've since realized that wider bars a.k.a. a wider grip allows you to engage more of your large chest & back muscles to push or pull on your bars - as opposed to using more of your biceps/triceps with narrow grip bars. A wider grip also increases your leverage while leaning your bike side to side between your legs, which has a multitude of applications. To make a long story short, if you're moving your hands further inward on your bars to accommodate more fingers on the lever, you're doing yourself a disservice.
P.S. None of that stuff I said really matters if using one less or other finger is too unsettling for you that it makes biking not as fun. There may be a way that anatomically and mechanically makes more sense, but adults riding bikes in circles through the woods doesn't make sense to a lot of people, so...