Can you try pressing the number 1 when top is running and see what happens.
Mr Unknown says it well.
top is saying "If you had one CPU core, it would be 57.5% busy."
But you have 16 cores. So this 57 number is spread out all over the place, with mysqld being multi-threaded and everything...
Kind of confusing. But I think this is because top would find it harder to display information for each core... imagine a top info line with 16 or more cores!
Also if you add up all the "%us" CPU time
1.7+1.0+2.3+1.0+7.4+11.1+15.1+8.7 = 48.3
48.3/16 = 3.01875
This is where the 3% number comes from...