I know this station and don't believe the mezzanine is the problem. There are two trains running perpendicularly. The "mezzanine" is the first floor with the blue line platform. Yes, additional stairs/elevators could help, as well as timing changes.

It's simply one of the busiest stations due to its location.

The huge mezzanine in the other stations such as at Pershing is mostly wasted space however.