Before proceeding make sure that you have at least UNA-v10.0.0-RC1

Actually I believe that something similar to star rating is possible to implement with reactions, you need to modify reactions icons and set the following 3 icons as possible reactions (but you need to do it in Developer module and be able to specify correct JSON string):

Also if you want to have star rating in comments only (convert them to reviews) then you can execute the following queries:

UPDATE `sys_objects_cmts` SET `ClassName` = 'BxTemplCmtsReviews' WHERE `Name` = 'bx_posts';
ALTER TABLE `bx_posts_posts` ADD `comments_avg` FLOAT NOT NULL;

Just replace "bx_posts" and "bx_posts_posts" with the module name and main module content table where you want to add star rating for comments.