Incredibly serious. The Smurfs were a major target of Satanic Panic profiteers, and I was banned from watching it by my parents (I usually ended up watching Saturday morning cartoons at my grandparents, and they were far less strict about it). Later on as an adult I had a conversation about my dad about it, and it boiled down to someone at church told him that Gargamel was actually another name for Satan, and since Gargamel uses black magic that the cartoon was indoctrinating children into Satanic dark magic practice. Again, not even slightly exaggerating here.
As you might guess "Gargamel" isn't a name for the devil*. And he is depicted as quite bad, so I don't know how one reads "bad guy uses bad magic to hurt cute blue small people who live in mushrooms" as "hey kids, worship Satan!" but that was the gist of it.
Gargamel's pet cat, Azrael, however is named for a supposed angel of death in the Islamic tradition (and, to a lesser extent, shows up in some Christian contexts in the middle ages). I have a theory that a game of telephone of some kind happened, and Azrael as an angel of death in medieval folklore blended into modernistic Satanic Panic circles as a name for a demon or the devil, and this is conflated with Gargamel; and somehow that is what got filtered into what my dad heard. I don't know exactly how widespread this was, but I remember reading a Texe Marrs book written in the 80s when I was a teenager in the 90s, he pretty much said that the devil is using everything to indoctrinate kids into Satanism and that there is a secret cabal of Luciferian New Agers running everything behind the scenes. Marrs was a total wackadoodle; but he like so many others convinced an entire generation of parents (at least) that everything in the world was coming to get their kids and bring them into some secret Satanic cult.
This was big business in many corners of the Evangelical and Fundamentalist world of the 70's and 80s. There were professional conmen who became incredibly famous not only in church circles, but even in secular circles as supposed "experts" on the occult. Mike Warnke is perhaps the most famous of these, he was everywhere, not just at churches and Christian conferences, but on mainstream broadcasts being interviewed by serious journalists.
This was a whole thing. And if you didn't grow up in that period of time, or if you happened to not grow up in a household where this was taken seriously, then bless you that's awesome. It was a weird time to be a kid.
-CryptoLutheran