If we narrow down “most crazy/stupid” to mean “most counterproductive to Germany’s war effort”, I’d agree that declaring war on the US is certainly at or near the top of the list. Germany was under no obligation to do so, and the last thing it neeeded was to add to its existing military problems by getting into an unnecessary war against the world’s greatest industrial power (and furthermore, against a nation which was separated from Germany by 3,000 miles of ocean, and thus which Germany’s land forces – which never even got across the English Channel in 1940 – could not invade).
The relevant article of the Tripartite Pact of 1940 – the one which said that “Japan, Germany, and Italy […] undertake to assist one another with all political, economic and military means if one of the Contracting Powers is attacked by a Power at present not involved in the European War or in the Japanese-Chinese conflict” – was clearly aimed at the US, but it didn’t apply after Pearl Harbor because it was Japan which had attacked the US instead of the other way around. And even if it had applied, Hitler could easily have ignored his Tripartite Pact obligations, given his long track record of tearing up treaties whenever it suited him. Note that the phrase “all political, economic and military means” did not specifically require the signatories to declare war; Hitler could easily have interpreted it to mean “all aid short of war” – which was Roosevelt’s own phrase for his pre-war policy of supplying arms and other assistance to Britain.
That said, a war with the US would probably have come sooner or later. Roosevelt’s “all aid short of war” policy supported Britain to such an extent that it severely pushed the boundaries of nominal neutrality. It included steps like helping Britain with the occupation of Iceland, and having US warships escort Britain-bound convoys as far east as Iceland. A few US warships even got torpedoed in the process – so at sea, prior to Pearl Harbor, the US and Germany were already in an undeclared shooting war. If Hitler hadn’t declared war against the US in December 1941, he probably wouldn’t have avoided a war with the US in the long term, but he might well have delayed it, thus gaining time which might have been used more productively.