"Become lumpy" essentially means forming clumps or bumps on a surface. Some of the common synonyms for this expression include "thicken", "coagulate", "clot", "congeal", and "curdle". These words are mostly applicable to various physical states of substances, such as liquids, solutions, or mixtures. For instance, when cream or sauce loses its smooth consistency and starts to form lumps, it can be described as thickening or clotting. Similarly, when a substance changes its state from a liquid to a gel-like texture, we use the words congeal or curdle. Overall, the key idea behind these synonyms is to describe the transformation of a homogeneous substance into a heterogeneous one with lumps or bumps.