If you think that, you don't read Punisher comics. Actually, imo, Frank is MORE resourceful than Batman. He doesn't know all the technowizardry crap that Batman knows, but when it comes to thinking on his feet (as opposed to analyzing chemicals or whipping up a reverse polarity catalytic frombwitzer or some other stupid thing a bored writer dreamed up), I've seen more from Frank than I've ever seen from Bruce.
Batman is very, VERY overrated. The Joker is even MORE overrated. That Punisher could and would kill him -- even if he KNEW Punisher was coming -- is so obvious as to be unworthy of further consideration. I'm more concerned with whether Frank could, in fact, also kill R'as al Ghul. I'm inclined to believe that he could, but it would be interesting and perhaps not so easy, assuming R'as knew he was coming.
Other people who could effortlessly kill Joker:
Elektra
Hitman (when he was alive)
Daredevil
Hell, even Catwoman could probably do it at least 5 out of 10 if she really wanted to. So could Harley, Joker's own main squeeze (in her own mind).