What's the word for something that's too direct and plain rather than poetic?

Solution 1:

As noted in comments, prosaic may work. Its senses include
• Pertaining to or having the characteristics of prose
• (of writing or speaking) Straightforward; matter-of-fact; lacking the feeling or elegance of poetry
• (usually of writing or speaking but also figurative) Overly plain or simple, to the point of being boring; humdrum

In the comments, and perhaps in the question, the third sense has been emphasized. But the first sense applies more properly and more widely. Even if poetry is written in “almost like plain English sentences”, it need not be at all unimaginative, banal, blah, boring, colorless, common, commonplace, dead, drab, dry, dull, everyday, flat, garden-variety, hackneyed, ho-hum, humdrum, lackluster, lifeless, literal, lowly, lusterless, mundane, ordinary, pablum, pabulum, pedestrian, plebeian, routine, square, stale, tame, tedious, trite, unexceptional, uninspiring, vanilla, or vapid. For example, free verse is a recognized poetic form; although at first glance much of it may look much like ordinary prose, some of it is readable poetry.

Solution 2:

Many lines of T S Eliot’s poetry, if taken in isolation, are banal: ‘I shall wear the bottoms of my trousers rolled’, ‘I can’t help it, she said, pulling a long face, / It’s them pills I took, to bring it off, she said’, ‘Home is where one starts from’. They gain their force from the context in which they are used. However, when line after line is like your made-up example, there’s no other word for it but prose. Being set out in a peculiar way on the page doesn't change that.