Game play runs perfectly so smooth but mouse has delay

I had the same problem. Here's what Paul (Admin) wrote to me, it seemed to help:

Are you experiencing mouse lag in the desktop and none in game?

If just on desktop please disable scroll inactive Windows feature.

Step 1: Go to Start > Settings > Devices. Step 2: Under the Mouse tab, locate to the Scroll inactive windows when I hover over them option and turn it off. Then, check if your issue is solved.

it doesnot works I tried it.
lagging is still there

For some users going into Moonlight settings (gear icon top right corner before connecting to the computer) and turning off Raw mouse input has seemed to fix the problem for them.