These are the questions that I ponder when I want to take a break from the law. My initial inquiry on Google ngrams suggests an affirmative answer to this question:
(I apologize for the overlap of the chart with the text on the right side of the page -- this seems to be an issue that Blogspot has not resolved at this time)
That search was not "case-insensitive" -- and a case-insensitive search gave me different results -- although my initial theory seems plausible:
A close look at the graphs, however, reveal that the decline in balderdash and tomfoolery seems to begin before any substantial rise in malarkey. It should also go without saying that I am aware that this type of search does not involve considerations of statistical significance and there may be crucial variables I am missing.
While trying to refine my investigation, I considered another common (though more vulgar) synonym that these charts do not include. While I will not mention that term here, beyond noting that it is commonly abbreviated as "B.S.," a link to an ngram comparison of that term with balderdash, tomfoolery, and malarkey is available here. While BS is now a far more commonly-used term than the others, the explosion in its use did not occur until around 1960 -- which was after the mid 1930s and onward decline in tomfoolery and balderdash. But a closer investigation of BS reveals that it was slightly more common than malarkey in malarkey's early years -- so a combination of BS and malarkey could have contributed to the decline of tomfoolery and balderdash.
As a disclaimer: if any readers think that the preceding discussion and charts have yielded a credible answer to the question I have posed, I would advise those readers to take a statistics class. But if readers think that this theory is one worth pursuing, I would advise them to do so. A Google Scholar search of all three terms malarkey balderdash tomfoolery together yielded only five results -- indicating that my thesis is rather novel.
Follow Up: "Dreck" is another variable that tracks malarkey's trend and which may be part of the explanation for the decline in tomfoolery and balderdash.