If something isn’t all it was cracked up to be, then it’s less than advertised, it’s a disappointment.
Crack began as the verb “to praise or boast” in the fifteenth century and today is often used as a noun.
For example, you might be cracked up by a good wisecrack.
In the South, a cracker is a feeble-minded braggart.
And if you’ve lost it, you’ve gone crackers.
No matter how you look at it, if it isn’t all it’s cracked up to be, you’ve been had.