prog, gg5, wisdom
daijr (sense 2): 仕組んだわけではないのに，ひどく滑稽で面白みがある・こと（さま）。「―な出来事」

I think "crowning achievement" should be reserved for 最高傑作. I don't think the x-ref is needed.
In meikyo and smk, only sense 2 is adj-na. I think the glosses are a bit off. It doesn't necessary refer to a blunder.
I don't think the "sarcastically" note is needed if we're tagging it as joc.

@@ -18,2 +17,0 @@
-<pos>&adj-na;</pos>
-<xref type="see" seq="2829836">最高傑作</xref>
@@ -22 +20,2 @@
-<gloss>crowning achievement</gloss>
+<gloss>chef d'oeuvre</gloss>
+<gloss>fine piece of work</gloss>
@@ -24,0 +24 @@
+<pos>&adj-na;</pos>
@@ -26 +25,0 @@
-<pos>&adj-na;</pos>
@@ -29,4 +28,4 @@
-<s_inf>sarcastically</s_inf>
-<gloss>amusing blunder</gloss>
-<gloss>funny mistake</gloss>
-<gloss>boner</gloss>
+<gloss>(unintentionally) hilarious</gloss>
+<gloss>absurd (e.g. blunder)</gloss>
+<gloss>outrageous</gloss>
+<gloss>wild</gloss>

GG5: a masterpiece; a crowning 「work [achievement]; a masterwork; a chef d'oeuvre 《pl. chefs d'oeuvre》; a magnum opus.

I think sense 1 deserves a bit more.

@@ -20 +20,3 @@
-<gloss>masterpiece(s)</gloss>
+<gloss>masterpiece</gloss>
+<gloss>masterwork</gloss>
+<gloss>crowning achievement</gloss>

傑作	3058151	  
傑作選	204246	  

definitely not adj-na usage:
傑作と	168025	  
傑作です	165294	  
傑作だ	145184

theoretically possible adj-na usage
傑作な	32824	  
minus not-actually-adj-na usage:
傑作なの	13205	  
傑作なん	5794	  
傑作など	926	
傑作なら	655	 
傑作なり	378	

actual adj-na usage:
傑作なもの	301	  
傑作なこと	286	

[2] seems dated (couldn't find real-world examples, did find a reference suggesting dated)
https://word-dictionary.jp/posts/4902
主に、ある人にハプニングが起こって皆で面白く楽しんでいる様子を表したり、ある人が滑稽な言動をして盛り上げたのを褒めたりするような意味で使います。
---->>> ただし、やや古めかしい言い方で現在はあまり使わません。

reversed [n] and [adj-na], added xref, removed "best work" (somewhat misleading, somewhat inaccurate).
added [dated] to [2] (last reference).

[adj-na] appears to actually be quite uncommon. You can google things like 傑作な作品, 傑作な歌, or 傑作な話.  They exist.  But this seems dwarfed by strict-noun usage.  Skimming briefly through yourei's 5000+ examples, I found no [adj-na] usage at all. (I only looked at a few pages)

reverso overwhelmingly prefers "masterpiece", sometimes masterpieces.  "best work" is only in a few E->J translations.  The "best" in "best work" gives a strong implication of "only one qualifies.", i.e. "the finest book he has ever written", but that is not the case.  Removing "best work" reduces this confusion.  "masterpiece" alone risks this "only one" connotation. Adding the extremely-common "最高傑作" makes it clear that "傑作" is not necessarily the artists finest work. It's just very good(another reason I dislike the "best" gloss here).  

傑作	3058438	85.5%
最高傑作	520759	14.5%  <-- that's a pretty incredible count

One of...
傑作の一つ	14858	68.4%
最高傑作の一つ	6851	31.6%

daijs:　たいへんできばえのよいこと・作品   

I wanted to drop "boner" as [dated] on our side, but it looks like the Japanese expression is probably also [dated], so marked as such.

@@ -16,0 +17 @@
+<pos>&n;</pos>
@@ -18,3 +19,2 @@
-<pos>&n;</pos>
-<gloss>masterpiece</gloss>
-<gloss>best work</gloss>
+<xref type="see" seq="2829836">最高傑作</xref>
+<gloss>masterpiece(s)</gloss>
@@ -22,0 +23 @@
+<pos>&n;</pos>
@@ -24 +24,0 @@
-<pos>&n;</pos>
@@ -25,0 +26 @@
+<misc>&dated;</misc>

@@ -24,0 +25 @@
+<misc>&joc;</misc>