When you state the tone of a piece, it is almost always up for debate, so as long as you choose something that makes sense, and you defend it well, you are usually OK.

I would state the the tone is bold.  Sonnets are supposed to be about love, and this one is, but it is not about beauty in the typical way.  The speaker is taking a risk by wooing his woman through insults.  He also sort of hands out a backhanded insult to other poets (including himself) when he says they are guilty of "false compare", in other words the women they write about could not possibly be as beautiful as they say.  His point to his woman is "I love you as you are."  Of course, there is a good bit of humor in the first 12 lines of the sonnet as well.  The humor only leaves at the end when he makes his true point known.

The tone is sometimes considered a subjective appraisal of a poem or other written work, since it can be something that a reader picks up on in a subtle way, and not necessarily spelled out in a clear and direct fashion.  The tone conveys the mood of the poem.

For me, the tone of sonnet 130 is mocking.  This is an interesting sonnet, in that even though the speaker is describing his lady love, he seems more concerned with slamming the cliched descriptions usually used to describe a love in poetry.  I've highlighted the cliched descriptions of eyes, lips and breasts that Shakespeare mocks in his opening lines.

My mistress's eyes are nothing like the sun;

Coral is far more red than her lips' red;

If snow be white, why then her breasts are dun;

If hairs be wires, black wires grow on her head.

This last description for me, tips the scale to a sarcastic mocking tone.  It's very hard to receive, as a reader, the description of "black wires" for hair from a poet who is sincerely pouring his love for his lady's beauty into his description.  His implication in choosing "black wires" seems to be the most opposite of whatever trite phrase most poets would use (Black as coal?  As a raven?) would be.

His reasoning in defying these celestial, cliched descriptions becomes clear when he  says:

I grant I never saw a goddess go;

My mistress, when she walks, treads on the ground.

His mocking, however, turns to a defiant and protective tone in the final couplet:

And yet, by heaven, I think my love as rare

As any she belied with false compare.

He makes it clear that he needs none of the false and trite comparisons to prove how deep and true his love is.

How does the tone of Sonnet 130 operate to engage the reader?

William Shakespeare's Sonnet 130 has often been called an anti-Petrarchan sonnet because it seems to be mocking the idea of the conceit in which the lover compares his love to qualities that are greatly admired.  But, instead of being a near goddess, the poet's love has many imperfections:

Coral is far more red than her lips' red;

If snow be whie, why then her breasts are dun;

If hairs be wires, black wires grow on her head.

Unlike the the love poem tradition as established by the Italian poet Petrarchan, the poet's comparison of his love does not illustrate his love's superiority to nature at all.  Instead, she is "nothing like" the beautiful productions of art or nature.

In another aberration from the Petrarchan form, at the ninth-line "turn," the formal point at which a sonnet typically introduces an antithesis to redirect the focus, the speaker continues his strange conceit, noting how music has a much more pleasing sound than his lover's voice.  Yet, he does love to hear her speak. 

And, then, in the eleventh and twelfth lines, the speaker casts of conventional descriptions of women as goddesses and writes that his mistress is earthy and earth-bound.  Finally, the couplet expresses the speaker's opinion, and the rhyming couplet explains the contrasts.  His lover is as beautiful as those that poets praise in hyperbole.  For in his subjective mind, she is rare, and the light-hearted and often humorous tone of the speaker engages the reader more than the fantastic conceits of the Petrarchan poets.