I disagree with creating a tag for this purpose.
Tags should be used to categorize content. Ideally, saying "I am an expert in tag-name" should pass the laugh test. We have a few meta tags (science-based, hard-science, reality-check, and perhaps a few others) which pass this test only because they aren't supposed to be used in isolation; a question can't be just science-based, though it can be for example science-based evolution. In this sense, our such tags are similar to for example Stack Overflow's language-lawyer tag, which also can't really be used in isolation.
However, nobody is going to say seriously and with a straight face "I am an expert in alien-did-it", or any variation thereof. Nor are they going to say "I am an expert in alien-did-it dragons".
Since you also said in a comment:
I'd expect better thought-out questions, which steam from serious attempt to build credible worlds. Instead we've recently been flooded with very silly "stuff happens for no reason, what then" which have blatantly no purpose other than having an idle chat. This is a very serious problem: if not addressed, it could easily kill this site.
Creating a tag for questions that you feel do not belong on the site in the first place is absolutely the wrong path to take. If anything, creating a tag indicates to people that the type of question covered by the tag is welcomed by the community, at least to some level. If you feel these types of questions are off topic, you should be flagging them as off topic, bring it up on Meta (we have a quite active community on our Meta, especially for a beta site), or bring it up in chat to try to gain a community consensus. All of these options are available to almost everyone with an account on the site, with the possible exception of chat which requires 20 reputation (which frankly isn't very hard to get on Worldbuilding; a single half-way-decent answer will likely get you there).
We have had significant discussions both in chat and here on Meta early during the beta period about what types of questions we accept, which has been summarized in the help center's page on what topics are on-topic for us; the on-topic tag here on Meta captures some of this. Our subject here at Worldbuilding, almost by definition, is going to be a lot less about right or wrong answers than, say, Stack Overflow, but that doesn't mean we don't have quality standards for content, only that our standards are different from those of the trilogy sites in particular.
One of the main ways in which Worldbuilding differs from many other sites is that we accept questions stating preconditions that cannot happen in the real world, which answers are expected to take into account. We have questions involving dragons, Ragnarök, teleportation, shapeshifters, miscellaneous mythical creatures like centaurs, magic and so on and so forth. We even have an actual question about unicorns. This allows for questions that handwave away how a situation came to be (so you don't even really need to say in your question "an alien did it" about how unicorns evolved in your world; if you state that unicorns exist, answerers are to assume that unicorns do exist), while asking for answers to what will happen in that situation.