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.