No, engagement metrics are certainly down because total number of ads are way up while the number of eyeballs watching them aren't increasing nearly as quickly. But it's actually really hard to measure engagement with digital ads since the vast majority of digital ads are display ads.

Furthermore, there's no real metric that can determine whether or not a user even looked at an ad: yes, the ad was loaded by the user's browser, but the user may not have scrolled down the page far enough to actually see the ad, or if they simply scrolled past it without actually looking at it. Because there is no good way to measure engagement, advertisers tend to create ads that promote being noticed over user engagement. Publishers have reacted by loading ads in different ways (e.g. multiple page articles, progressive page loading, etc.) to support higher ad rates on premium content.

There was a recent trend in last few years that showed mobile ads had higher engagement levels, though many in the industry now think that a substantial portion of this is due to "mis-clicks" on mobile ads. The trend is moving away from click-throughs as a measure of engagement as a result of unscrupulous publishers building sites that make it reeeeeally easy to mis-click on an ad. Regardless, measurement is a huge problem in the digital space when it comes to engagement; though the trends relating to engagement on a per-ad basis are almost certainly declining simply due to the increase in quantity of ad inventory and the reduction in transactional friction involved in advertising over the last 10 years.

It's called banner blindness. "Engagement" of ads is dropping like an anchor yet prevalence of ads is skyrocketing. So you could say the mean effectiveness is really dropping because of this.