I was just wondering as I saw this question Unable to install Ubuntu 14.04.3 or Linux Mint, are half-Minty questions like this allowed or should they be closed even if they are also as about Ubuntu?
A question needs to be about Ubuntu to be on-topic. Whether or not it is also about Mint doesn't really matter.
The problem with most Linux Mint questions is that they are not about Ubuntu. Many people assume that because Linux Mint derives from Ubuntu and is similar to it in many ways that a question about Linux Mint is within the scope of our site just because it's asking about Mint. That is not the case.
However, mentioning Linux Mint or stating that a problem also happens in it does not cause a question that is already on-topic for some other reason to become off-topic. It's not that the presence of Mint is sufficient to make a question off-topic, it's that the presence of Ubuntu is necessary to make a question on-topic. If the OP is using Ubuntu and is also using (or has used) other OSes that are mentioned, that doesn't make it any less of an Ubuntu question.
There are other ways a question can involve another OS like Mint and be on-topic. Someone might be using Mint and want to install Ubuntu. An Ubuntu user might be trying to create a Samba share and mention that it will be accessed from other machines running other OSes, such as Windows and Mint. This is all fine.
Sometimes when someone mentions other OSes that also have a problem, it doesn't add anything to the question and could be removed. However, I'd be careful about editing out such information and often avoid it, because such information often serves to establish that the problem in not particular to the specific installation of Ubuntu that they are running. That is, it's like if they mentioned they had the problem on another Ubuntu system. This is not in any way Mint specific. For example, someone might ask about a device that is supposed to work in Ubuntu and Windows but stopped working in Ubuntu after they upgraded to a newer release, and they might mention that the device mysteriously never worked on their Windows system. Whether or not that information is useful would depend on the situation, but it would not likely affect whether or not the question is on-topic.
Finally, one caveat: sometimes Mint users mention a specific release of Ubuntu just to tell us that it is the release from which their Mint release derives. This is a case of the wrong assumption I mentioned above--the assumption that Mint must be on-topic because it's based on Ubuntu. Most of the time it's easy to figure out what they mean, and you can always ask them if you're not sure. They might not be sure either, but you can ask them to show the output of
lsb_release -a (or, if you prefer, just
lsb_release -d) "on the Ubuntu system."