In Harry's second year, Ginny Weasley wrote him a particularly horrible poem. That in itself isn't surprising, since she was infatuated with him.
His eyes are as green as a fresh pickled toad
His hair is as dark as a blackboard
I wish he was mine, he's really divine
The hero who conquered the Dark Lord
(Harry Potter and the Chamber of Secrets, page 238)
Why does Ginny call Lord Voldemort the Dark Lord? Only the Death Eaters and his other supporters would call him that. People who are against him call him You-Know-Who, or He-Who-Must-Not-Be-Named, except the few who dare to speak his name.
Ginny was being possessed by Tom Riddle through the diary, but would he really change the way she would refer to him? He told Harry that he was sick of having to listen to Ginny. He obviously wasn't interested in her life, he just needed someone vulnerable enough to do his bidding. I don't think Tom Riddle would want to pay attention to what she's doing any more than he has to, or that he'd care enough about a silly little love poem enough to change the wording of it to use his preferred title. When he's not trying to possess her and she isn't writing in the diary, presumably he'd leave her to live her Harry-obsession filled life by herself, not concerning himself with it further.
The other things about the poem, like the overly fawning tone and the unfortunate nature of comparing Harry's eyes to pickled toads and his hair to a blackboard, are easily explained by her being a young infatuated schoolgirl. So why does she call Lord Voldemort "the Dark Lord" when it's a term of respect for him only used by his supporters? Ginny usually calls him You-Know-Who, which is a common way for average wizards to refer to him. Also, "You-Know-Who" has the same number of syllables as "the Dark Lord", and is easy to rhyme with. Both terms are equally as easy to write into a poem.