I like this idea - knowing the general reasons why people are disliking a place would be very helpful to developers.
Maybe an even better way of executing this would be to have, rather than a modal window, a pop-out element appearing when you select a rating - something like:
The rating will be registered immediately when you click the thumbs up/down, and if you don’t submit a reason, your rating will just be counted towards the default reason. That way, if you simply don’t find the game to your tastes, or you don’t feel like submitting a more specific reason (which I imagine would apply to a lot of ROBLOX users), you’re not forced to click through a second time to submit the rating.
Reason for this is that I think having a big modal appear every time you try to leave a rating might just put some people off from using the rating system altogether (particularly kids who might be intimidated by such an interface). The pop-out element I suggest might not be as effective at gathering specific reasons for ratings, but it also won’t impact the ease of using the rating system.