Will changing 1/2" PEX from Main Line to 3/4" PEX increase flow to Sprinkler Heads
Pressure is pressure when you eliminate flow, add flow and the distance and you will end up with a lower pressure and poor performance like you are experiencing.

On the long runs increasing the plumbing from 1/2 to 3/4 will improve the sprinkler performance.
This will be true as long as your pump can keep up with the demand.

If you have several different sprinklers / spigots running at the same time your usage may exceed the capability of your pump. With large systems we segment the number of sprinkler heads that are allowed to run at any one time and cycle them for best performance.

So yes increasing the supply plumbing especially on the long runs may be what you need.