Exactly!! That is the key to realize the tale has at least two layers.
First is the one we've all learnt when we were kids and that is pretty much similar to "not everything is gold which is sparkling" - you'd better have something that's useful than something that only looks good.
But, exactly like you said, that leaves the question open: so, the mouse had done all the hard work for them, why are they now unhappy? I think that is because they tried to misuse the golden egg - the precious part is the golden eggshell, not the yolk. You could do something likemy preciousa golden ring or golden coins from the gold that the eggshell contains. But the simple-minded couple armed with the consciousness not very much higher than that of a simple speckled hen, decided that the important part is the yolk, and the golden eggshell is just something nice to look at, nothing more. So, those hungry individuals lamented over the spoiled yolk not ready to realize that the golden eggshell is there for them (to make golden stuff) for their chicken-level brain does not allow them to think out-of-the-eggshell, so to speak. Good thing they were consoled by the creature (whose cognitive processes is just their match) which teaches them go on not-to-thinking and not-to-worrying.
Ergo: is not worth it to cast pearls before swine.![]()