On every sheet that has comments in it, Google Sheets shows the total number of comments in a green box in the tab at the bottom of the screen:
Is there any way to hide it?
From my knowledge, there is no way to hide this comment counter. However, be aware the flag will only count open comments and not total comments on the tab. This means if you resolve open comments in the Sheet, you will notice the counter going down and eventually disappearing when there are no more comments left.
You can see all comments on a sheet by right-clicking on the tab and selecting "View Comments". You can also cycle through all comments on the sheet/tab by using Ctrl + ALT + N to go to the next comment, or Ctrl + ALT + P to go the the previous comment. Finally, you can navigate through the comments via Comment History.
This way you should be able to eventually find and resolve all open comments as a workaround to get rid of the counter.