While watching Return of the Jedi someone watching it with me said “Jabba the Hutt is silly, there is no way such a creature could evolve”. Now there are a great number of silly things in that movie, so it is strange that that was the thing that broke this person’s suspension of disbelief.
I don’t see the evolution of the Hutt as impossible. We only have a single example and know very little of the context. Is Jabba a typical example, particularly old or obese? It is not hard to imagine an old, overweight billionaire (how about Carlos Slim, currently the world’s richest person), supposedly very successful people in our modern world. Can you imagine them being as successful in the wild? How about them feasting on a wild animal they just killed, blood running down the wrinkle grooves. Doesn’t seem quite right. Survival of the fittest may drive evolution, but that doesn’t mean the definition of fittest stays constant. The forces of evolution that shaped us are no longer in effect (or at least not to the same extent). I don’t see why this wouldn’t be the case in any sufficiently advanced civilisation.