Do you get lots of fog in So Cal?
Depends on the season/temp, of course, but SoCal fog areas tend to be...
(1) ...in passages between areas with different temps/altitudes (Ex: Cajon Pass in fall/winter often has heavy fog between the cold, high elevation High Desert and temperate, lower elevation Inland Empire). When these passages get fog, it's usually pretty dense.
(2) ...in foothill areas where the cool, marine layer builds up overnight, usually dissipating in the morning by around 9am (Ex: Chino Hills/Diamond Bar/Brea/Anaheim Hills area, Pasadena/Burbank/Griffith Park area, Whittier/La Habra Heights area, Devore/Rancho Cucamonga/Upland area, etc). These areas get mild to moderate fog the most consistently; pretty much daily in the Fall.
(3) ...occasionally in the slightly inland coastal areas in the late evening during the Fall (Irvine, Costa Mesa, Laguna Niguel/Hills, Vista/Bonsall, etc). This doesn't happen a ton in these areas, so locals aren't as familiar with fog driving etiquette, leading to the occasional fender bender.
As far as San Diego, fog tends to be more prevalant in North County (MCB Camp Pendleton, Oceanside, Carlsbad, Encinitas/Leucadia).
So, while fog is not super common in SoCal on the whole, if one lives in any of the above areas, fog lamps can be a helpful upgrade for your daily drive.