A pig on a frozen pond is clumsy and awkward, which is how the person in the simile sentence appears when they are on the ground.
Context: "Even though she's big, she's not very brave-and when she's on the ground, she's clumsy." Claim: "She's big, but she's not very brave-and on the ground she's like a pig on a frozen pond."
Considering the context, is the claim true, false, or unknown?