What *kind* of monologue? One of my all-time favourite books, Nicholas Stuart Gray’s fantasy The Stone Cage, begins with a monologue by a very vain cat, and it’s hilarious. Iirc The Murder of Roger Ackroyd also begins with a monologue.
To pull it off, you need to be very good at writing inner dialogue. The tone needs to be lively and interesting, and either engaging, funny or sinister, and it shouldn’t be more than a couple of paragraphs. Otherwise, you’ll just turn readers off.