Shakespeare's Sonnet 130 is indeed, like his other sonnets, written in iambic pentameter: a regular metrical form in which there are five "feet" (a pair of syllables in which the first is unstressed and the second is stressed) in every line. This poem, however, offers an excellent example of the fact that iambic pentameter need not mean that each line slavishly follows its precedent in terms of rhythm and emphasis. While iambic pentameter usually means an unstressed syllable followed by a stressed syllable, repeated five times over the course of a line, there can be variations on this theme, as we find in this sonnet.
It is interesting that Shakespeare should choose to use a reversal of the more common rhythm of iambic pentameter in this particular poem, which criticizes the romantic sonnet tradition as a whole. The speaker is saying that to compare his mistress's eyes to the sun would be hyperbolic and ridiculous; as he defies convention in terms of theme, he also plays with convention in terms of iambic pentameter.
The first line uses traditional iambic pentameter stresses:
My MIStress' EYES are NOTHing LIKE the SUN
In the second line, however, we can hear how this meter is reversed:
CORal is FAR more RED than HER lips' RED
Looking at other lines in the sonnet, we can see that Shakespeare toys even more daringly with iambic pentameter at times:
If SNOW be WHITE, WHY then her BREASTS are DUN
In this line, two stressed syllables follow each other.
If you read the poem aloud, it is an interesting exercise to sound out where the natural emphases fall as you read. In doing this, we can see that Shakespeare stretches the convention of iambic pentameter distinctly in this sonnet, just as the sonnet itself stretches the convention of romantic poetry.