Viewing reaction responders on mobile devices (hover / mouse-over)
On a PC, just hovering your mouse over the reaction buttons can show you who agrees with your post or reacted to it. If you want to see who agreed or voted-up your post on a mobile device, here's some tips that may work for you. These tips have been tested on Android devices using the Chrome browser. This may not work on all devices. I was not able to get either method to work on my iPhone SE using the Safari browser, but I did not try the Chrome browser on my iPhone.
Put your browser in desktop mode and long-press the reaction button (hold down for at least one second) to bring up the context menu--you may be able to briefly see the people who reacted to your post before the menu comes up--then press the back button to dismiss the context menu and you should now be able to see who reacted to your post.
Put your browser in desktop mode and click on a reaction button, but leave your mouse cursor over the button. You should be able to see the people who reacted to your post. Click again if you want to toggle your reaction to remove your vote.
Comments
In case it wasn't clear above, step 2 above is done with a wireless mouse connected to your mobile device. Step 1 is done with your finger.
Doing what with your finger
Ask Ricky.