"Sonnet 130" in particular is an excellent example of the way in which Shakespeare ridiculed elements of Petrarchan sonnets. Some of the contemporary poets writing at the time of Shakespeare would use fashionable but highly exaggerated metaphors to describe the women that they loved. I am sure you can imagine the kind of metaphors they used: your cheeks are roses, when I see you I burn with passion and so on. This will give you a general idea of the kind of elements that such poetry contained. Such metaphors are known as "conceits" and can be traced back to Petrach, but by 1600 they had become risible and overused. Shakespeare thus explicitly writes in a way that mocks such conceits by emphasising the all-too normal nature of his beloved, as the following quote demonstrates:

My mistress' eyes are nothing like the sun,
Coral is far more red than her lips' red...

Shakespeare thus deliberately takes such conceits and turns them on their head, pointing out the way in which his mistress is normal and not a highly-idealised perception of beauty. Note that in "Sonnet 18," the speaker does idealise the beauty of his beloved, but not through elaborate and exaggerated conceits. Instead, the comparison of his beloved with summer allows the beauty of the object of the poem to be established and immortalised without the need for such conceits.