Most questions that are on-topic here, are on-topic there too.
But it doesn't go the other way around. For example, questions about Linux Mint are on-topic there, but not here.
And if you post Ubuntu-specific questions there, you might get a comment saying "You might find more expertise on Ask Ubuntu".
I recommend that you ask Ubuntu questions here. (Even if you think the same question may apply to Mint, Elementary, etc.) If your question is about Ubuntu (or Xubuntu, Kubuntu, etc.), ask it here.
Here are some quotes from the tag descriptions for Ubuntu and Kubuntu at Unix and Linux SE:
Note that http://askubuntu.com is specifically dedicated to Ubuntu questions.
Questions for Kubuntu are probably better asked at http://askubuntu.com/, the stackexchange site for Ubuntu.