"Idiom poem" is not a formal literary term or category. It is thus up to personal interpretation, but it could either be any poem that makes use of idioms as its central focus or any poem written in a non-standard dialect of a language.
According to the Literary Devices website, an idiom is a phrase that has a recognized meaning different from its literal meaning or a dialect restricted to a particular geographical area or association of people. An idiom poem could make frequent use of commonly recognized idioms like "rub him the wrong way" or "out of the blue." It could also be written as a representation of a certain dialect of English such as the famous dialects of New York or Louisiana.