As you are undoubtedly aware, the speaker is comparing the importance of being a mother ("the biggest thing on earth") with that of being learned (e.g., highly educated) or a graduate.
To emphasize the difference of that importance, the speaker describes the latter attributes as "very good and respectable" things and immediately devalues them by saying that twelve such things make a dozen.
(Imagine a situation where someone wins some award and someone says to that person: "Receiving that award is a very good and respectable thing, and I'm sure that if you were to walk into a coffee shop with that award and a five dollar bill, you could even buy coffee!" Same idea.)
In US English, there is a similar expression (a bit more direct). Instead of "...and twelve of them make a dozen," we'd say: "...but such things are a dime a dozen." (Where a dime - being ten cents - is not of much value at all.)
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To emphasize the difference of that importance, the speaker describes the latter attributes as "very good and respectable" things and immediately devalues them by saying that twelve such things make a dozen.
(Imagine a situation where someone wins some award and someone says to that person: "Receiving that award is a very good and respectable thing, and I'm sure that if you were to walk into a coffee shop with that award and a five dollar bill, you could even buy coffee!" Same idea.)
In US English, there is a similar expression (a bit more direct). Instead of "...and twelve of them make a dozen," we'd say: "...but such things are a dime a dozen." (Where a dime - being ten cents - is not of much value at all.)
Hope this helps.
Cheers...