CPU usage represents how much of the CPU is being used by a program. So if you are using 20%, it is only using 1/5 of the potential of the CPU. This could mean its not utilizing all of the cores. The problem with a lot of CPUs over 4 Cores for gaming, is that many cores can be left unused, wasting potential. It's not necessarily a bad thing, it really just depends on the processor. I would say the Ryzen 1600 WOULD bottleneck to some degree, as it's a low power processor not really oriented for gaming. But it shouldn't be noticeable to the extent that you can't pair it with the 1070.
And as stated above: the term bottleneck is misused a lot. I would say that the CPU won't bottleneck the GPU, but the processor might not perform very well in some games.