The narrator is really just saying that the mother looked like she was very light (the light that means the opposite of heavy), but is using a metaphor within a metaphor to say it.
The first metaphor is describing how light she was. She was as light as being able to be blown away by the breeze. The second metaphor is describing how something can be blown away by the breeze. It can be like a dandelion gone to seed.
For the specific imagery of a dandelion going to seed see here, and particularly:
And, as many a child discovers to their delight, when a dandelion sets seed, the flower (actually, hundreds of tiny florets) turns into a mass of seeds known as a dandelion clock. Each seed is suspended from a parachute-like stalk — easily released by a puff of breath.