The Flynn effect reversal has a pretty simple (but unpopular) explanation that has nothing to do with microplastics.
But more concretely, there is evidence that conservatives out reproduce liberals in the US and in part it is related to the urban - country divide and that could also be related to both pollution, but also likely cost of living: https://ifstudies.org/blog/the-conservative-fertility-advant...
An example would be people who go onto Twixxer to find out what they are supposed to be mad about today. I understand that there are legitimate issues which need a spotlight put on them and social media can be a great tool for that. But it also thrives on that level of anger and energy, so there is a decent amount of less-than-dire issues being flogged just for engagement.
If enough people are just entertaining their brain away/being directed by an algorithm more often than not, I can see this moving the needle on general intelligence since "the brain is a muscle" and not using it means you can lose it over time.
AFAIK there are some studies that show that dysgenics are at least not the only effect in play (e.g. this study https://www.pnas.org/doi/full/10.1073/pnas.1718793115). But I am far from an expert on the topic.