First of all, as Zanna already pointed out in her answer, WSL is on topic here as decided by this meta discussion:

Do we consider "Ubuntu on Windows 10" the same as an official Ubuntu flavour? Do we support it? To what extent?

In general, adding a non-Ubuntu repository to your system doesn't bar you from asking questions here. If that were so, anyone who had dared add a PPA, for example, wouldn't be able to ask here! The general rule is that if you installed an official Ubuntu flavor, then your system is on topic. That's why we allow questions about non-Ubuntu kernels, about desktop environments—such as cinnamon—not used as the default environment of any Ubuntu flavor, about non-default shells and software in general.

However, if your problem is directly related to the mint repositories you added; if, for example, there is a compatibility issue with the packages in the repository and you can't install something because you have the Mint rpos, then I would say your problem is off topic and you should probably ask on Super User or Unix & Linux.

If, on the other hand, yours is a general question about using WSL, then the fact that you happen to have added an extra repository doesn't mean that your question is off topic, no.

First of all, as Zanna already pointed out in her answer, WSL is on topic here as decided by this meta discussion:

Do we consider "Ubuntu on Windows 10" the same as an official Ubuntu flavour? Do we support it? To what extent?

In general, adding a non-Ubuntu repository to your system doesn't bar you from asking questions here. If that were so, anyone who had dared add a PPA, for example, wouldn't be able to ask here! The general rule is that if you installed an official Ubuntu flavor, then your system is on topic. That's why we allow questions about non-Ubuntu kernels, about desktop environments—such as cinnamon—not used as the default environment of any Ubuntu flavor, about non-default shells and software in general.

However, if your problem is directly related to the mint repositories you added; if, for example, there is a compatibility issue with the packages in the repository and you can't install something because you have the Mint repos, then I would say your problem is off topic and you should probably ask on Super User or Unix & Linux.

If, on the other hand, yours is a general question about using WSL, then the fact that you happen to have added an extra repository doesn't mean that your question is off topic, no.