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.
But 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.