A mix of two concepts:
"LA" is presumably not the Lennox and Addington revered by Avril Lavigne fans worldwide but
Los Angeles, a podunk California town which attracts would-be actresses and models from all over. Many have perfect faces.
Conversely, Oakland is a city in the Bay Area in which many who have been priced out of San
Francisco by the Silicon Valley rabble and the dot-com bubble have taken refuge. A black man and a rapper, Sir Mix-A-Lot would be looking for ghetto booty - a booty that is large, round, and
bootylicious.
So, presumably, the best of both worlds.
Seattle-
based rapper Sir Mix-A-Lot saw the Oakland girls might not have been China doll
beautiful like the A-list cinema stars from Burbank, but were particularly endowed in another area.
This phrase, LA face with an Oakland booty, is talking about a female who has both.