Don't be too harsh to these poems until they're typed. I always think typescript lends some sort of certainty: at least, if the things are bad then, they appear to be bad with conviction.

Thomas is wryly describing the psychological shift that occurs when a draft moves from handwritten manuscript to typed copy. Typescript, with its uniform letters and apparent finality, can make tentative lines feel “settled,” even when the writing is weak. The joke cuts both ways: typing can falsely dignify mediocre work, but it also clarifies flaws by presenting them with a kind of formal authority—“bad with conviction.” The remark reflects a working poet’s awareness that presentation affects judgment, and that revision is partly a battle against the deceptive confidence conferred by neatness and finish.