"Is {blank} a serious religion?" In dealing with alternative faiths, I see this question a lot.
Religions aren't serious. It's up to people to take them seriously or not. So whether a religion is being approached seriously can only be judged on a person-by-person basis...and even then its an awkward subject. The believer may describe himself as serious even thought he looks pretty goofy to outsiders. Does that count as serious?
My rule of thumb is anyone who honestly believes in their religion is being serious about it.
One hiccup to all this: there are people who ascribe to religions that they specifically describe as not serious (such as a the cult of the Flying Spaghetti Monster). But such things are definitely the exception rather than the rule.