If you add a co-owner - you'll be subject to gift tax which is exactly the same as the estate tax. There's one benefit however: gift tax has a $14K exemption a year. So you might save a bit of a tax by giving the gift now instead of having it inherited later, but on the other hand - it will be you paying the tax now instead of the heir later.
Of course, all the issues raised in the comments about co-ownership are valid - liability, you won't be able to change your mind (you can change your will any time, but you can't take a gift back after given), etc.
Reading the comments, I feel that some more elaboration is needed.
Note: I'm neither a lawyer nor a tax professional. Get a legal advice from an estate attorney licensed in your state. Get a tax advice from an EA or a CPA licensed in your state. I only write my opinions based on my understanding of how things are.
Re the gift tax: the tax applies to the value given. So if you have a $100K Tesla S, and you add an owner to split the ownership in half - the gift tax will apply to the $50K given (half of the value of the car), and the $36K above the exemption will be taxed (assuming you didn't give that person anything else throughout the year). You can mitigate the tax using the $5.25M lifetime exemption, or you can pay the tax and keep the exemption for the estate tax later.
The tax is assessed against the FMV: Fair Market Value of the gift. So if instead of a brand new Tesla S you give away half of the 1993 Honda Civic that is currently worth about $3K, you'll have $1.5K subject to tax (which is way below the $14K gift tax exemption), and it doesn't matter that 20 years ago the car was worth $15K.
Re the liability: insurance company will only issue insurance to the person(s) on the title. I know that because I checked, and I'm in California as well. So if you give away half of the car - you'll have to be tied to that person wrt the car insurance. You'll have to have insurance good enough to cover both of you, not just you.
If there's a loan on the car, there's a lien on it. You can't change the title (ownership) until the lien is removed. The bank will only remove it when the loan is repaid.
This paragraph that you wrote show that you don't understand how the estate and probate work:
I ask because it seems that adding a person to a title is a relatively
simple process. Removing a dead person from a joint title looks
relatively simple also. I don't see anything about taxes or
inheritance policies, just vehicle registration fees. However, if the
person wasn't on the title already prior to the death... things appear
to get complicated and involve inheritance taxes/etc.
The fact that the car is co-owned doesn't make removing the dead person simple. Unless the title is held jointly with the right of survivorship, your part of ownership will have to go through the probate court. This kind of title is common for married couples wrt real estate, but I don't think you can do it for cars. Maybe you can, ask at the DMV.
Probate means that the court will assign ownership per your will/inheritance laws, and it is a lengthy process that can go on for months (and years, if there are disputes) during which the car title can not be changed. It will be held by the estate executor until the court approves the heirs to take possession. There are ways to avoid this, but you have to talk to a estate attorney about this.
Estate planning is a complicated task and there are people who do it for a living. You should talk to these people, they're called "estate attorneys". They can provide you with a proper legal advice, build a proper structure and suggest the proper ways to manage and own your property in order to minimize problems and taxes later. Make sure to also talk with a tax adviser and make sure the attorney and the tax adviser are on the same page. Many times attorneys don't take tax considerations in the account (although estate attorneys specifically probably do, since its a very large portion of their work).