Having lots of cores isn't likely to matter much for mobile users, simply because most mobile apps are neither optimized for parallelization nor CPU-hungry in the first place. If 4 cores are good enough, it doesn't matter if you add hundreds more. That said, there may be specific application that might benefit, such as computer vision.
In special cases you can get better performance with more, but less beefy cores (graphics is a prime example), but in general a few powerful cores performs better. The main reason is because communication among cores is hard and inefficient, so only embarrassingly parallel programs work well when divided among many cores. Plus the speedup you get from parallelize a program is minor in most cases. See Amdahl's law[1] for more on this topic.
Also, I'm not an expert in this area, but I have some familiarity with it. So hopefully someone with a bit more experience can come and confirm (or refute) what I've written.