"Not too bad" for "very good"

• Figure of speech like "no mean feat"
• Little child in stories making understatement
• Understatement for effect
• Figure of speech that employs ironic understatement
• Figure of speech like "not unlike"
• Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
• using ___: “Mary had no giant lamb / Its fleece was not unwhite”

Wiktionary Word definitions in Wiktionary
n. (context rhetoric English) A figure of speech in which the speaker emphasizes the magnitude of a statement by denying its opposite; a figure of speech in which understatement is used with negation to express a positive attribute; a form of irony

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
rhetorical figure in which an affirmative is expressed by the negative of its opposite, from Greek litotes , literally "plainness, simplicity," from litos "smooth, plain, small, meager," from PIE root *(s)lei- "slimy, sticky, slippery" (hence "smooth"); ...