妈 = 女 (woman) + 马 (horse), where 马 (mǎ) only lends the sound. So it's 'the woman you call mā'. Pure and simple: the woman radical tells the meaning, the horse tells the sound.
妈妈 (māma) is doubled, like English 'mama' — and that's no accident. Across almost all the world's languages, 'mother' starts with an m-sound, because 'ma' is one of the very first sounds a baby can make. Chinese kids say 妈 before they can write a single stroke. Note the tone: 妈 (mā, flat) is mom, but 马 (mǎ) is horse and 骂 (mà) is to scold — tones matter a lot here!
A woman (女) — the one you call 'ma'. MOM.
妈 (mā) mom, 麻 (má) hemp, 马 (mǎ) horse, 骂 (mà) to scold — same sound, four tones, four totally different words. The classic demo of why tones matter.