The shake modifier has a (Jumps per second) “value”, and an (Amplitude) “value”
Both of those values can be key-framed with different settings along the time-line.. and that can be used to increase or decrease the shake effect.
Once you have some Key-frames in the time-line, you can change the “tweener type to Oscillate or Noise or Bezier,..but those will only control the transitions between the key-frame values,.
Creating a “smoother shake” would depend on the Amplitude values and the number of jumps per second. (relative to the frame rate).
You can also adjust the Jumps per second, and Amplitude by expanding out the Modifier tracks in the sequencer, and in the graph editor, this gives you a finer level of value adjustments, than the numeric input fields in the modifier tab.
You can also create a Camera shake by using a “Target helper object” and using a Point at Modifier on the Camera to point at the target helper, and then applying the “Shake” modifier to the Target helper object, ...instead of on the Camera.
Hope it helps