There’s a mismatch on the verbs in this example. It can be fixed in one of two ways.
Reading a book would be boring only if you lacked imagination.
Reading a book is boring only if you lack imagination.
Remember what I said a while back about how only‘s position in a sentence can affect the meaning of that sentence? Let’s look at what happens, then, if we leave only where we found it (while still fixing the verbs): Reading a book is only boring if you lack imagination. Here, the sentence says reading is nothing but boring for a person with no imagination. Compare/contrast that with the previous versions, which say the only people who find reading boring are those who lack imagination.
Imagination should not be taken literally here, by the way. Reading books doesn’t have to be boring simply because the reader has aphantasia and is unable to form mental pictures. Mercenary Proofreader’s clone-sibling has aphantasia, and he writes books. The English language needs another word for the ability to conceive things that are not real and present that doesn’t imply visuals as the primary or only way of conceiving those things.
For that matter, lack of imagination (in the generic sense) doesn’t have to make reading uninteresting, either. Maybe the person who created the meme this example came from made the error of equating book with novel — y’know, fiction. (There are degrees of fiction-ness, too — some people read a lot of mainstream fiction but cannot stand sci-fi/fantasy because it’s “not real.” An individual with no interest in imaginary/made-up things can still find enjoyment in reading a book; the world is full of good nonfiction, too. So maybe it’s a particular lack of imagination that assumes lack of imagination means lack of interest in reading. 🙂