ABB-style descriptive adjective:
矮 (short) + reduplicated
墩 (a stout stump/block). The doubled syllable adds vivid, slightly cute color — typical of spoken Mandarin. Other ABB adjectives in this family:
胖乎乎 (pànghūhū, plump),
圆滚滚 (yuángǔngǔn, roly-poly).