1 hour 35 mins could be tight, depending on a number of factors including whether your inbound flight is on time, how busy immigration is at the time you arrive, and even whether you're nearer to the front of the back of the plane.
If your inbound flight is delayed to the point that you miss your connection then it is the responsibility of the inbound carrier (ie, the one that made you late!) to arrange alternative flights. The problem is that I'm guessing that's Lufthansa and they don't have a large presence at San Diego Airport!
If you simply miss your connection because it takes longer to get through immigration than expected, then technically the airline isn't responsible - but presuming you're on a single ticket then they generally will still look after you and arrange an alternative - but again it may be difficult to actually find Lufthansa staff to do that.
However one of the areas that US airlines excel over many other airlines in the world is the existence of an unpublished rule that is normally termed the "flat tire rule". If you arrive at the airport late and miss your flight (due to, say, a "flat tire" on the way to the airport) then they will either confirm you onto a later flight or at least put you standby if no seats are available.
The exact rule varies from airline to airline, and generally requires you to be at the airport no later than 2 hours after your flight was due to depart, although for something like a delayed inbound flight they will generally be even more flexible than that.
All up, 1 hour 35 should be enough presuming your inbound flight isn't delayed, and you don't loiter on the way (especially getting to immigration!), but if you do happen to miss your connecting flight you will be looked after, one way or another.
It's also worth noting that there's a direct flight FRA-LAX departing around the same time as your flight FRA-SAN. Although Lufthansa doesn't generally allow changing flights, it may be worth asking at check-in to see if they could move you to the direct flight and remove this risk entirely!