Fractals are a pure mathematical idea/construct which generates for example (not only of course) Mandelbrot sets or Koch curves. We live in a 4 dimensional universe and not in 3.141592653589...dimensions or in any other rational/irrational numbers of dimensions (string theory dimension do not count here because nothing can leak into it because they would be (if exiting) much too small)
. If we would live in a space with a different dimension we would probably already have recognized it, e.g. Newtons Law of Gravitation would simply not work physically that way we see each day. Besides this there had been implicitly searches for that hidden dimensions by means of altering Newtons Law (which is quadratically reciprocal in lenght) by guessing a 1/r^(2 + very tiny number)-law - with the effect that this 'very tiny number' is not distinguishable different from zero (with current methods). So a fractal idea would reflect simply a similarity and nothing deeply hidden in nature as far as I can see.
There are repeating structures in nature (and it is possible to assign simply a fractal dimension to physical structure) but this is then only true up to specific point (think of trees for example or a coast line of Britain which is of course not infinit of length). At least any repeating structure would have an end at the Planck Lenght (~10^-35m) for any physical structure which is not true for a fractal law.