Titles are usually pretty important in poetry, and they can also be kind of fun. This title fits the bill on both counts.
This simple, one-word title does a couple important things. First of all it puts a clear image in our minds: we can really see that slimy toad. While there are some amphibian lovers for whom this single word title might conjure up some warm, fuzzy feelings, the majority of readers are going to picture that toad and feel, at best, indifferent or, quite possibly, a little grossed out.
The second thing the title accomplishes is an important setup. We think we are about to read a poem about a slimy toad. Then, we read the poem's first line and begin to realize this poem isn't really about toads at all. The result is that kind of off-balance feeling we get when we are expecting one thing and are given something else entirely.
There is also a little anger or resentment that comes from not getting what we're expecting. It's like when you show up at the cafeteria on tater-tot day and they serve you creamed spinach. How do you feel? That's right. You are enraged. Granted, that is a very extreme example, and the negative feelings you experience from being duped by this title are no doubt subtler, but they are there nonetheless and that's just how Phil wants it. But why does Larkin want us to feel off-balance and a little miffed? Good question.
The end result is that we carry these off-balance feelings, and maybe even a little anger and resentment, with us as we read through the poem. Once that happens, Larkin has us right where he wants us. We are in the perfect emotional state to experience the poem and the ideas it explores. Well played, Phil. Well played.