David Icke created his shape shifting, inter-dimensional lizard rulers "to make conspiracy theorists look like tin foil wearing loons."
... so you think he made that bit up to intentionally discredit his other theories and make "them all sound like nutters"?
That makes no sense at all, in an early chapter of 'The Biggest Secret' he says that his agent and publisher advised him not to 'mention the lizards' at all because they thought it would be detrimental to the success of the book and thought that it could ruin his career. I think the chapter is even called "Don't mention the lizards!"
I have absolutely no doubt he believes what he says when he talks about them.... and it's not that crazy, plenty of people believe we live in a multi-dimensional universe and plenty of people believe there are other 'beings' out there. If you do then it's not a massive leap I guess.
I'm far from convinced and I don't see any real evidence of it being true ... but on the other hand, if we do ever have contact with beings from somewhere else, I'm pretty convinced they won't be human and after seeing 'simple' lifeforms like the octopus and squid change their appearance and colour, I don't think them having the ability to 'shape-shift' is beyond the realms of possibility either.
The flat earth theories are a completely different thing because that is fairly easily disproved, unless you dismiss science almost completely.