Since you're all on one ticket, you don't need to worry. I wouldn't plan anything right after your scheduled San Diego arrival, but I wouldn't worry too much
Finnair use the American Airlines terminal in JFK, terminal 8, which should help a little bit. When you get off the plain, look for a member of AA staff (may also be dressed as OneWorld), who'll hopefully be on/near the jetty. They will be handing out Orange express connection passes to anyone on a very tight connection. Your 90 minutes might not count as that tight, so they may not have one for you, but find them and ask especially if your inbound flight is in any way late. (The express connection is only available on through tickets, where they know about both your flights). Head down to immigration, and if you got the express pass use that to take the shorter queue.
To avoid delays at immigration, make sure you filled out the CBP customs declaration form on the plane, make sure you filled out correctly, make sure you have details of where you're staying, how you're getting there etc.
Next, head to baggage claim. You must collect all your checked luggage at your first US port of entry, as Helsinki doesn't have CBP pre-clearance. When you have your bags, go through customs giving them your form stamped by immigration. Delays possible here, so make sure you don't have any food / haven't been near wildlife or any of the other things that can trigger a mandatory inspection.
On exiting customs, head to the baggage re-drop desks. They'll take your bags (which should be tagged to SAN) from you, and feed them back into the baggage system. If you have been heavily delayed, at customs or immigration, ask the people at the baggage re-drop desks where to go for rebooking. (Probably upstairs, not certain)
With bags re-dropped, head upstairs to departures. Use your express connections pass to get the shorter queue to TSA security. Go through security. Make sure you follow the instructions carefully, especially around liquids in special bags separate, laptops and tablets out in their own tray, shoes off etc. Ensure nothing in your pockets. You want to avoid bags getting pulled for secondary inspection.
Now, all being well, head for your gate, and board! If you get delayed at security, find an AA customer service desk after security for re-booking.
If all goes well, you should be fine to make the flight. If you have problems/delays, AA will re-book you as it's a through ticket. (They even do it for some oneworld separate tickets which is nice). However, if you're delayed at immigration or customs or security, while they will rebook you, they won't provide hotels or food, as the delay isn't there fault. Here's where good travel insurance can be a big help! (But get stuff in righting/printed about the cause of the delay if you'll need to claim on your insurance)
AA will get you to SAN, maybe not at the time expected (but probably), just stay calm and do your best!
