Is it just the natural effect of Reddit becoming mainstream, or is mean IQ dropping fast?
It's mostly just the result of Reddit (and the internet as a whole) having become mainstream.
The internet in general has gone to shit. Low effort memes, pun chains, childish name-calling, childish sarcastic mockery of the other side....everything that gets upvoted appeals to the lowest common denominator.
If you want to spread a message to a large audience, you're much better off putting two lines of impact font on an image that strawman the opposite side of the issue than you are constructing a well thought out argument with proper citation. Attention spans are far too low for anyone to bother reading the latter.
Another thing to consider is that most web traffic is now from mobile users...and nobody wants to read paragraphs of text on a phone, let alone type that much. They want 10 words of impact font on an image that they can laugh at while sipping their 5 dollar frappuccino at starbucks.
"Meme culture" is another thing that drove down the overall quality of posts pretty much everywhere. Back in my day, memes were generally specific to a certain website. You had YTMND memes, 4chan memes, Something Awful memes, etc. If you posted a meme from one website to another, they'd tell you to get the fuck out. Now everything has become homogenized into one "meme culture" and every website, every forum, every subreddit uses the same memes and just changes the text on them to suit the political agenda or topic.