I don’t think that’s it, actually. I collect old cookbooks and household manuals, and my impression is that during the late 19th/early 20th century, big formal or “fancy” meals underwent a shift from more regional dishes (such as those terrapins) to a more uniform and largely French cuisine. Home cooking also leaned increasingly French in that era (or more like pseudo-French - white sauce on EVERYTHING!), but more importantly the early 20th century saw a shift from a dependency on heavy Victorian staples to lighter, more quickly prepared, and more “modern” styles.
Part of this was the change in the servant situation, with lower-middle class women increasingly doing their own cooking, part was a growing feeling that meals revolving around boiled or baked meat and fatty starches were unheathy, and part was the availability of certain foods via the modern food industry. They certainly aided and abetted some abominable cooking directly through “101 things you can do with Jello”-style marketing, but also just by making certain foods more accessible, whether by making formerly labor-intensive foods easy (bottled mayonnaise) or by making seasonal foods available year-round (canned pineapple and tomatoes).
The roots of horrible 60’s cooking are visible even in the 1910’s; ham/chicken/whatever salads (on lettuce, not as sandwiches) shot up in visibility, and persisted as a “dainty” feminine lunch into the 1930’s, once they went from a demonstration of the cook’s skill and effort (in making mayonnaise by hand) to something that still had upper-class cachet but without the effort. Fresh pineapple was expensive and seasonal, but the canned version allowed a luxurious and exotic addition to a fruit salad anytime (possibly with mayonnaise on top - no, I’m not kidding).
So on the one hand you have widely available commercial versions of “dainty” foods with upper-class connotations by virtue of expense (canned asparagus, canned turtle soup) or labor (bottled mayonnaise, canned aspic), and on the other an obsession with “new” and “modern” foods, and with “light” and “dainty” foods; Jello caught on in part because it was different from what previous generations had eaten, as well as because it was easy to prepare (as long as you had an icebox), and “light” rather than “heavy” (like pies or puddings).
So of course the pendulum swung too far in that direction, with the “new” and “exotic” dishes (without introducing many actual new ingredients or preparation methods) and the creative use of packaged foods, until that in turn became passé, and traditional, regional and “lower class” foods started to come back.
I have no opinion on the historic state of American beer (I don’t drink any), but from the statements in this thread I might speculate that something similar happened with it; I’m getting the impression that beer of the 60’s and 70’s was light, highly filtered and pasteurized, and probably very different from the beer of the 1900’s. Maybe that lightness and uniformity itself was once perceived as modern and desirable, like the fine, soft, white, hygienically packaged bread that was once so much better than the coarse home-baked loaf.