If your information is already prepared in the way you're showing and you're just missing the percentage, there's a super straightforward trick for you.
You can edit the custom HTML of the infowindows, which means that besides selecting which columns and titles are going to be shown there, you can also play with them a bit.
The workaround is easy: you just need to write a '%' character after your column names in the custom HTML code of your infowindows. Notice that infowindows use Mustache templates, so if you see {{column_name}} this means that the information of that column for your specific point will be added there when clicked.
With this idea, you can just use "{{column_name}}%" in order to insert a percentage symbol at the end.
Let me add this visually for it to be more straightforward:
