There's no particular reason to assume that other relay services might not mutate the body in some other way, nor that smee.io's mutation is going to remain the same forever, so we aren't going to attempt to optimise this specifically for smee.io. The only safe way for a "relay service" to deal with the body of a signed webhook is to leave it untouched; I'd consider this a bug in smee.io.
There's no particular reason to assume that other relay services might not mutate the body in some other way, nor that smee.io's mutation is going to remain the same forever, so we aren't going to attempt to optimise this specifically for smee.io. The only safe way for a "relay service" to deal with the body of a signed webhook is to leave it untouched; I'd consider this a bug in smee.io.