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.
