Under California law, if one spouse wants a divorce, they get it. Ignoring even an improperly served petition, will not prevent the divorce from moving forward. You will do yourself more harm than good if you don't respond. In most divorce cases, there is one spouse who wants the divorce done last year and the other spouse who doesn't want it at all. This creates a lot of conflict. I would suggest that you consult an attorney to understand your options for divorce (process, results, settlement, etc) and that you speak with a divorce coach to help you through the emotional process. Divorce has been identified as the second most traumatic event in a persons's life (second only to the death of a loved one). You will go through the same phases of grief as when a loved one passes away. If you wish to keep this amicable, I would suggest you look into either mediation or a collaborative process. Either way, you do need to prepare yourself for the divorce.
I'm not sure the basis for your saying that service was done improperly, but if your wife, or her attorney, files a proof of service which reflects that service was made correctly, and nobody tells the court otherwise, the case will proceed.
If they file a proof of service stating that you were served they could move forward and get a default if you do not respond. You could file a motion to quash service claiming that it was defective but that is a waste of time and money as they will then just reserve you.
You need to file a response and move forward to protect your rights.
You have not stated how the service was improper. If you were served with the stamped paperwork by anyone over the age of 18 other than your wife. The divorce can and will continue if you don't respond. You wife will be able to proceed by default. It is in your best interests to file a Response and then work on reconciling the marriage after you rights have been protected.
The response above does not form an attorney-client relationship. This answer may or may not apply to you and should not be relied upon as legal advice.