There's no tag because it hasn't been needed yet, but it could be made
There's no dc or difficulty-class tag because nobody has made one. There's no reason other than that for them to not exist. If someone wanted to make it, it would exist. A new tag can easily be made when a question is asked just by adding it to the question, provided that the user has 300 reputation.
300 reputation is not very much, so the level of trust in a user that the community needs for tag-creation is quite low. That says a lot about how much of a non-issue it is to create new tags. Our tag system is a folksonomy—the tags that exist reflect what people use, not some idealised categorization system that sprang from the forehead of Athena.
There are perhaps other tags that might fit, yes. If one of the existing tags covers the question's contents well, then perhaps a tag like difficulty-class wouldn't be needed. Its hard to tell without seeing your question.
But there's also no reason to shoehorn an almost-right tag into a question when a more appropriate tag could be created. If your hypothetical question was very much about DCs in general and naturally covered more than just skills¹ (e.g., spell save DCs have nothing to do with skills), then a tag like skills wouldn't naturally describe the question and a tag more like difficulty-class would make sense to create.
Most importantly, this is entirely hypothetical right now: a tag doesn't exist until at least one question uses it. Every tag was created after someone asked a question that looked like it needed a new tag to cover it properly. So ask your question, and if someone with ≥300 rep thinks a tag like difficulty-class is called for, it will get created!
- Assuming it's broader than just the topic of skills because it is naturally so, not just because it's multiple questions posted together.