What does “Clearasil-scented grammatical sloth” in casual American speech mean?

Yes, you have it. 'Clearasil-scented' is intended to mean teenagers, and their 'grammatical sloth' their laziness in holding to standards.

To address your particular questions, no, it is not about an alien, medication-like smell. It is a figure of speech, metonymy, clearasil being used by teenagers mostly.

It is not a set-phrase at all. If anyone else used it, it would probably be considered esthetic plagiarism. Meaning it wouldn't really be plagiarism, but if used without quotes, totally uncool ('esthetic plagiarism' is the first time that phrase has been used, but you can use it if you like, just give me credit the first time).

"Is “Clearasil-scented” a popular phrase?" NO, the author invented it on the spot for this sentence. It is scathingly bitter.

NOTE interestingly the product Clearasil does not have much smell, so it's actually not really that ingenious a construction.

NOTE, it's relatively common to talk dismissively about teenagers / kids as smelling like "bubble gum". (You can google around and find constructions about that.)

Almost certainly the writer had that in mind but wanted to choose something more bitter.

does “Clearasil-scented grammatical sloth” mean immature usage of language?

NO -- it simply means BAD use of language, by teenagers.

Note that (as I've mentioned to you many times on this forum), in all the English-speaking countries I know of, we're going through a thing where the quality of language is decreasing, and staggeringly so.

The author seems to particularly be attacking young people with regard to this problem; IMO the author is wrong -- all age groups are equally at fault.