The push to have a satanic monument erected seems rather childish to me. If it was wrong in the first place for the Ten Commandments monument to be erected, they should focus on having it removed. Trying to have their own monument erected seems like a position of "two wrongs equals a right". It's like noticing that other motorists are speeding without being fined, so you speed up as well, even though you had no desire to speed prior. If they can do it, so should we.
Having said that, I doubt it is a serious proposal.
If you read the linked articles, you'll find the openly stated goal of the Satanic Temple is to point out the ultimate result of saying it's okay to put explicitly religious monuments in public spaces: everyone
is entitled to do so. In the US, it's fundamental that there can't be government endorsement of a particular faith, so the courts have consistently agreed that either you allow everyone
to express their religious sentiments, or no one
So in that sense, yes, it's serious. This kind of "religious creep" into public life is potentially very
serious, magic. Cases like this highlight both the absurdity of that and the necessity of keeping religion the f*ck out
of places where it has no business being.