Tone and attitude are important in determining the meaning of any written work, not just poetry.
Tone/attitude is the same thing as feeling or emotion.
When analyzing anything literary (and well, visual art probably counts here too) it is important to understand how the author's feelings combine with his or her mode/style of writing (rhetorical techniques) to create a purpose - which ultimately is creating a feeling or emotional response for the reader.
Typically, when you can determine how the author feels, you can determine what it is he or she is trying to say... what MESSAGE he or she is trying to send.
When analyzing poetry, tone and attitude are directly linked to the theme of the poem, so a true understanding of a poem must also include an understanding and exploration of that poem's tone and attitude. For example, Marge Piercy's "Barbie Doll" is about a girl who does not have an ideal body type and wants to be someone who she is not. There are hints of sarcasm throughout the poem that are woven through elements of despair. The speaker's attitude towards ideal beauty standards is clearly negative since the poem shows a girl's life ruined by societal pressure to look a certain way. Tone and attitude are assessed through the context of the poem, diction, syntax and figurative language.