This sentence is humorously ironic for several reasons. First, the speaker is portraying themselves as a well-meaning charity worker, an archetype typically associated with virtue, integrity, and selflessness. However, the purpose of this "collection" is not altruistic but rather to supply a tool for an abused vulgar phrase that encourages someone to engage in self-pleasure as a form of insult. The gap between the implied goodwill and the actual vulgar intent is where the irony lies.
Secondly, we have the charitable cause itself. Charitable donations are usually meant to assist those in need, often providing items necessary for survival or comfort, like food, clothes, or toys. In this context, it's funny — albeit in a dark, inflammatory way — to consider used sex toys as items of necessity. Adding another layer of humor, is the amusing innuendo in the phrase "Toys for Twats," a parody of various holiday toy drives like Toys for Tots.
Another element of humor stems from the absurdity of the concept. The idea of collecting used sex toys, let alone donating them, is in itself uncomfortable, bizarre, and not the kind of thing that happens in any usual circumstance. This absurdity is what makes the sentence humorous to some people.
The irony also comes from the tone of the sentence. The speaker starts off with a formal salutation, "Good afternoon," as if embarking on a serious, benevolent conversation. However, the politeness and formality quickly descend into vulgarity, creating an ironic contrast between conventional charity call and the actual crude proposition.
The hilarious nature of this sentence also lies in its blunt, no-nonsense way of addressing a common sentiment. While the obscenity of the phrase is clear, it satirically conveys a feeling that many people experience — the desire to tell aggressive, argumentative, or obnoxious individuals to "go away" in less polite terms.
Lastly, the sentence expresses an artful form of sarcasm bordering on slapstick humor. The visual of a door-to-door charity collector asking for not just money or goods, but used sex toys, and that too for an arguably offensive cause, portrays a picture so utterly ludicrous it can extract laughter even from those annoyed by its vulgarity.
GOOD AFTERNOON I M COLLECTING USED DILDOS AND VIBRATORS TO DONATE TO ALL THE BITCHES THAT NEED TO GO FUCK THEMSELVES Toys for Twats