[…] almost any slopeDefeats her modest hopes. (6-7)
As a synonym of "humble," "modest" has positive connotations—unpresumptuous, reasonable, discreet. But here the word is linked with "defeats," triggering other, less positive, connotations of "modest"—meek, unsure, unassertive. Come on, Turtle, time to play "Queen of the Mountain." It's a competitive game, and you don't have to be so polite!
[…] never imagining some lotteryWill change her load of pottery to wings. (12-13)
In Aesop's tale, the tortoise never expected to grow wings and fly across the finish line. But as she plodded along, don't you think she still had the goal in mind, still had hope that winning was not totally out of the realm of possibility? The fact that Ryan's turtle is too humble to imagine a dramatic reversal in her situation does not necessarily mean that she is too meek to contemplate more measured success. Or does it?
Her only levity is patience, (14)
One synonym for "patience" is "humility," so "patience" is not an unexpected word to find near the end of the poem. But we keep bumping up against the odd pairing of "patience" with "levity," which means "cheerful humor or lightheartedness." There's no way around the implication that humility (patience) has its own mysterious joys and rewards.