I remember the comic-book story this episode was based on. It was one of those giant-sized SUPERMAN ANNUALS. And, I found this episode to be a mostly faithful adaptation of same!
Sure, some elements in the original story had to be omitted by the scriptwriters. But, that's only because this was going to be a half-hour one-shot! Not the first half of a two-parter. In hindsight, maybe it should have been. But, then again, everyone's hindsight is 20/20. Isn't it?
As for Superman's birthday being (quite literally) universally known? I don't think the writers were referring to his actual birth on Krypton. Or, even, the anniversary of his adoption by the Kents! More likely, Wonder Woman and Batman--and, yes, even Mongul--were observing the anniversary of when Superman first appeared on the public scene. The "birth," symblically speaking, of Earth's first modern superhero!*
*As opposed to Spy Smasher and other costumed "mystery-men" of the 1940's .*
Considering everything the Justice League has done on behalf of certain extra-terrestrials since its formation, I don't find it strange, at all, that some of them might be prone to showing their gratitude by sending its arguably most powerful member a gift in commemoration of that anniversary.
Thus, in my estimation, this episode is a perfect (as in completely flawless) ten!