The flyswatter in fact doesn't even deliver electricity to the object it hits at all - it is only a fairly powerful electric discharge - very harmful to electronics and small bugs, quite safe to pretty much anything else.
You can definitely use it safely for cbt, no guarantees on how much or little it will actually hurt though.
Also, let me clear up a few common misconceptions about the dangers of electricity in general:
The real danger is current, not voltage. You can subject yourself to tens of thousands of volts and live to tell the tale - although depending on where and for how long, you will have massive burn injuries - which in their own right CAN be lethal in the extreme.
As for current, it takes extremely little to stop your heart if in fact the current travels through that region. Which in turn fortunately takes a lot to happen. Current chooses the path of the least resistance, which is very rarely across your chest. The most dangerous move you can make to make that happen is to short a circuit using both hands, but even then it rarely takes a path through the heart. Still, I wouldn't call 10% of something killing you good odds lol.
As far as batteries vs. mains, that is also often misunderstood. DC batteries, even at far lower voltages, are capable of delivering extreme amounts of currents in a short time, often far more so than AC mains, and is ironically therefore more dangerous in terms of cardiac arrest than AC mains are.
As is so often the case, what hurts the most isn't necessarily most dangerous
The obvious bottom line though, as always: It can kill you. Messing around with electricity is a real danger if you don't know what you're doing. And even safe electro play can have other, non-lethal consequences such as nerve damage.