米 pictures grains of rice scattered around a central node. As the indexing radical it marks 糕 as a grain-based food and groups it with 粉, 粒, 粽, 糖, 粥 - cakes, flours and starchy preparations all share this signpost.
羔 supplies the sound gāo cleanly. The lamb-on-fire imagery is purely phonetic here - 糕 is a steamed rice cake, no meat involved. Same phonetic family includes 高 (gāo, tall) and 羹 (gēng, broth).
