I don't think so. I use my first knuckle (the inside of course) of my right thumb, and it doesn't really hurt.

Don't take this as gospel, cause i'm normally a guitar player, but that's my experience with it'
If Rock is a lifestyle, then Metal's an addiction

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Nah...I've been playing bass for a long time and my thumb never really hurt. Granted slapping isn't my main style but when i got into it i never felt discomfort...except when i accidently hit my bridge....that blows. just feel it around and find the sweet spot on your bass.
your thumb shouldn't end up hurting unless you slap really hard and go for a long time. however the fingers that you pop with (pointer and middle fingers) will hurt pretty bad when you first start out. like, really bad. I couldn't play slap for more than 15 minutes when i started. But now i can go for a long ass time before it starts to hurt.

2008 American Standard Fender Jazz
Ibanez SRX300
Ashdown MAG 300 C410T + 1x15
SansAmp 3-Channel Bass Driver D.I.
Nah ive been working on a lot of songs with slapping for about a year now, and even in the beginning my thumb never hurt from it. Although i have gotten a blood blister from the finger i pop with after playing for a while.
]One Among The Fence