This is a platitude, a hackneyed phrase which means that being unaware of some things is a great pleasure. Most people would be inclined to agree that there are things about which they'd much prefer to be in complete ignorance, but the statement naturally rebounds on itself as a joke. The built in definition of bliss is an inevitable metaphor.
Examples of Ignorance is Bliss:
Ignorance may be bliss, but for a swimming instructor, that's a bit too blissful for my tastes.|
If ignorance is bliss, you must be the happiest person on Earth.
If you're that ignorant, why aren't you more blissful?