Would it be considered a malapropism to substitute a word or portion of a word in a phrase with another word or portion of a word, generally rhyming with the original word, and generally considered an offensive replacement? For example, the term "Election Day" substituted with "Erection Day" or the company name "Pizza Hut" to "Pizza Slut"?
I shall attempt an answer based on my having read Sheridan's great work, The Rivals, many, many years ago at school.
Of course, one of the play's characters, the redoubtable Mrs. Malaprop, has delighted generations of readers with a mirthful reminder that the English language is sometimes a source of amusement but also a potential provider of a sine qua non (there I go again, spouting Latin on ELU) for all who wish to cast aspersions on those of us who routinely blunder our way through the language with malapropisms aplenty. Even the late Christopher Hitchins, who harnessed the English language to become the media's outstanding polemicist and iconoclast of his generation, occasionally let fly with a chuckle-inducing malapropism. No one would think less of him, surely!
And I can't agree that a malapropism is always an "offensive replacement", as per the OP's comment. On the contrary, I see no reason to describe any malapropism per se as having this characteristic.
EDIT: Moreover, the OP's "Pizza Slut" for Pizza Hut and "Erection Day" for Election Day are examples of malapropisms which I find amusing and the kind of comical absurdity in keeping with the equally absurd and comical Mrs. Malaprop in Sheridan's 'Comedy of Errors'. I am not offended at all nor would I expect anyone to be other than those of a prissy outlook on life.
Malapropism: "The usually unintentionally humorous misuse or distortion of a word or phrase; especially: the use of a word sounding somewhat like the one intended but ludicrously wrong in the context". (Merriam-Webster)