"There are worlds who are simulations" !== "Most / all worlds are simulations". The second assumption is possible but by no means implied by the first. If we could create such complex simulations, it would prove only that it's possible that we live in one.
On the bottom end of the stack, for such thought exercises ordered by descending charm, it could be that we live in a simulation created by someone / something that exists in the real reality, and we aren't even in a sophisticated pinnacle of technology achievement like a supercomputer, but in the real reality equivalent of a cheap videogame given away as a real reality equivalent of swag: "Universe 2000, Acme Inc special edition".
Ouch. What a depressing thought.
Probably make a decent sci-fi short story. Now if only I had the talent or patience for writing...
--edit-- My own thoughts were along the lines of - humans decide we're probably in a simulation, and try to figure out how we could attempt communication with the simulators. To do so we decide to build our own simulation, then run it at double speed and watch how they try to communicate with us. Then recurse...
If you ever read a story like that, you'll know someone with the same idea but more motivation than me wrote it :)
Afai(!)k, there might as well not be a top layer but infinite nesting -- or, even more charmingly, endless recursion, each simulation containing not another layer, but another iteration of itself. This at least would eliminate the embarrassment of being "less real" than the above layer.
edited: initially linked to the wrong movie