On August 27 of last year, a poster on the main site asked "What does the idiom “funny as hell” mean?" Four days later, the question was closed for a familiar reason:
Please include the research you’ve done, or consider if your question suits our English Language Learners site better. Questions that can be answered using commonly-available references are off-topic.
As currently worded, the "funny as hell" question reads as follows:
I really don't understand this idiom, hell is supposed to be a horrible place. I understand the saying which is present in dictionaries “hot as hell”, but I could not find “funny as hell” in any online reference.
An editor added the sentence "I could not find 'funny as hell' in any online reference" on November 28, 2018, presumably in hopes of signaling to potential close voters that an explanation of the exact phrase "funny as hell" (the particular phrase that the poster was interested in) was not easy to find online.
I acknowledge that some EL&U participants believe that editing a question to help it avoid closure for lack of research constitutes cheating, defacement of the original post, vandalism, outrageous presumptuousness, insupportable interference with a poster's (1) right to ask questions that contain close-worthy flaws and (2) status as the only person with the necessary insight to resuscitate them, etc. This has, I believe, always been a minority view at EL&U—and it seems to be fundamentally at odds with any rationale that might explain the site's decision to let people other than the poster edit a poster's question—but nevertheless it persists.
In any event, the question was closed on August 31, 2018, for the "Please include the research you've done..." reason, notwithstanding the question's containing the "could not find 'funny as hell' in any online reference" showing of research.
Despite its closure, the question has attracted (to this point) 7,156 page views in just 8½ months. Evidently, the poster isn't the only person wandering the internet who wonders why people say "funny as hell." The number of views suggests that the question is of broad interest to site visitors. It is also clearly a question about English usage. And finally, it involves a more subtle issue than the close voters may have realized—namely, that "[blank] as hell" is a surprisingly slippery construction.
The highest-voted answer points out that "as hell" frequently serves as an intensifier in situations where, objectively, hell makes no sense as a simile for the thing being intensified—"happy as hell," say, or "devout as hell." But in some cases, the expression does work as a simile: "hot as hell," for example, is perfectly coherent as a simile (as the OP points out); and readers of Dante could make a case for "cold as hell" as a simile, too. Likewise, "cruel as hell," "treacherous as hell," "murky as hell," "painful as hell" (or "hurts like hell"), etc., are far from irrational as straight similes.
It is in this light, I think, that we should view the poster's curiosity about why "funny as hell" works in English even though it obviously fails as a coherent simile. I think it's a good question of broad interest, and I ask that site participants who have the power to vote on such questions consider voting to reopen it.