MongoDB just utilizing one CPU core
There is no such notion as CPU "traffic". A single thread runs on a single Core. Always. While a single core can execute multiple threads and even multiple processes, a single thread can not be split to be executed among multiple cores. That would require the system to understand the purpose of the program, which would be a pretty frightening thing.

So let us assume you have 4 threads and 4 Cores. Now let us assume 3 of those threads do not have much to do (one is logging, one is listening for configuration changes and one accepts new connections and spawn additional threads dealing with those connections), but the other one really has to do some computation. The core the computational intensive thread is attached to will have a higher utilization. And this behavior is even intended – or would you want the system to slow down in accepting new connections just because somebody makes a computational intensive request?