I wanted to know if a question about Skype or related software would be considered on-topic.
If not, what applications are?
All applications that run on the PC would be on-topic here. The main stipulation is that it has to be about software running on a PC.
Asking questions about the Skype app on an Android tablet or phone or iOS device would be off-topic unless the problem is something odd that happens only in relation to your computer.
So, Skype for Destops or it's ModernUI counterpart would be okay, Skype for Android would not.
If you want to know what questions are and are not on-topic then you can find that information in our help centre.