Slap bass is often learned as forearm rotation. The thumb is upright, and the rotating forearm hits the thumb joint against the string. (The exception is Flea, who points the thumb down but still rotates.) This technique works fine.

There is another way where the thumb does not rotate but pushes down onto the string and then onto the next one. This is a rest stroke (likeĀ  classical guitar). Larry Graham does this, as does Victor Wotton. Who will also double-thumb.

Rotation flows, especially with alternate pops, and creates a nice equilibrium. The disadvantage is hitting other strings with the upright thumb. The no-rotation rest stroke controls the strings better and, I think, is louder when required.

It's always worth looking to learn authentic techniques to extend your musical palette.