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.
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.