No, not at all. I am no giraffe myself. But the heights given on actors' bios are always exaggerated, unless the actor is actually a giant, in which case they are reduced. He appears to be a lot shorter than 5'8", judged by pictures of him with other actors of comparable height.
The imagery of The Joker, for me, is quite important. His frame is quite specific. He should be tall, lank and looming. I realise that this doesn't describe either Jack Nicholson or Heath Ledger, but it would be nice to finally see someone who is "dead-on" for the part.