Mafalda's speaker opens with things other people say: your mom is great, I wish I had her, why don't you talk to her? Those compliments set up the gap the rest of the poem is about, the difference between how the mother looks from outside and how she is at home. The speaker says she loves her mother but sometimes hates her, and the poem never tries to smooth that contradiction away.
The repeated "If they knew" works like a slow confession. Each time, the speaker adds another detail: the mother drops everything for her boyfriend, gives her kids little attention, and treats him as more important. The speaker also says her mother broke her trust and is one of the reasons she dislikes herself. The language is plain and the lines are short, which makes the hurt feel honest rather than dramatic.
The mood is sad, lonely, and a little defensive. The speaker feels misunderstood, since others may think she's ungrateful. Yet the ending is fair-minded: maybe her mother doesn't realize it, but she still failed her.