Bad slap technique??

There are NUMEROUS (!) ways to play slap bass, but I would recommend to follow the following advice:

"When slapping or popping, attack by rotating your hand at the wrist(!) only, moving your thumb toward the stings when slapping and pulling your finger(s) away from the strings when popping."

From here:
Slap Bass - Techniques, Adjustments and Tricks to Develop Your Tone

 

Playing slap bass requires a light touch, economy of motion, and very little physical effort. You should not feel any pain, fatigue, or soreness. You definitely shouldn't "feel the burn" like you have been lifting weights! Les always looks relaxed and cool as a cucumber, even when he is playing very complex music.

Here is an excellent "how to play slap" video:

 

Everyone has a different approach on slap techniques especially because now there are several ways do do it.
The first thing that I can suggest is the same of the previous post, economy of the movements, work on your thumb with the wrist action instead the forearm strength, that will give speed and control of your dynamics. I always suggest a thumb calibration exercise.
On open strings, playing quarter, eighth notes, sixteenth notes in sequence on all your strings and focus to get the same dynamics and volume being focused to don't hit other strings, than play groove combination, even just simple octaves intervals thumb and plucks, and try to get the same dynamics and volume on the plucked and thumbed notes.
Usually a good spot to hit the strings for a classical/standard slap is the very end of your fingerboard and last but not least, a medium low, or low action is much better.
Here some of my videos where you can get some suggestions to how play the basic movements and where on the fingerboards. Always work for the better and comfortable way for you, look to the others and find yours
If you need any further help, just let me know.
Let's groove. Ciao

 

You just have to find what works for you. Look at the varied thumb techniques between Larry Graham and Flea. They hit with different parts of their thumb. I can slap just like Flea, but I can't get anywhere near Larry Grahams style.

 

I see three style ...
Pointing down thumb
Straight thumb
Up thumb

Pointing down is like Flea, interesting but limited, more often than not, the bass is low and angled
Straight is like Les Claypool, fast and allow for a lot of the more extrem slap technique, most often than not the bass is at a normal height to avoir any bend
Up thumb is like Mark King, can be very fast but you need the bass quite high to get there