Unfortunately this has been rejected as per the official list of rejected suggestions:

The Official List of Rejected Suggestions wrote:
4.2 Dislike button or a project rating system
Generally, a user's first project is something rather simple, such as a test project, a remix, or a project made using a tutorial. Because this project is simple, other users may give the project low ratings since it is not advanced. With low enough ratings, the creator might be discouraged enough to stop using Scratch. Moreover, constructive feedback is encouraged on Scratch. A dislike button may show the creator how many people disliked their project, but it tells nothing about what the creator could do better. If you do not like a project, you could choose not to love it, or you could leave a respectful and constructive comment telling the user how to improve, in addition to saying what you like about the project.
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With that, a dislike button is rejected for anything, including projects and comments. However, a like button for comments is NOT rejected. You can discuss it on this topic.
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Also, even if it was constructive, little Timmy would be sad seeing his project got a thumbs down.