坨 is used for soft, irregular, shapeless masses - mud, dough, butter, wax, etc. It is colloquial and somewhat informal. Using it for something unpleasant (一坨屎, a pile of crap) is very common in casual speech.
register
坨 is casual and sometimes crude. In formal writing, prefer 块 (kuài, piece/chunk) or 团 (tuán, ball/lump) depending on context.
Earth on the left puts 坨 with mounds, lumps and clods of soil - words like 块, 堆 and 坡. A 坨 is essentially a shapeless lump, so the earth radical evokes the original image of a mass of dirt or clay.
Provides the sound, drifting from tā to tuó. It joins a small phonetic series with 驼, 陀 and 沱 that all share this stem. The pronoun meaning of 它 plays no part in the lump-of-stuff sense.
