As long as the satirist is willing to laugh at himself first, nobody should be safe from his satire. Freedom of speech should be protected at all costs, and the act of censoring what a cartoonist can or cannot draw is unacceptable in a free-thinking society. An artist often draws on his personal experiences, including his relationships with others, therefore representing past acquaintances like this is entirely acceptable subject matter for his pen or paintbrush.
During a relationship there are many situations that are deemed private by one or both parties. Depicting these moments could be very offensive to your ex. It is important to consider the feelings of others when drawing something so public as a cartoon. Depending on its content it could also be considered harassment or defamation.
I don't think it's a good idea to include an ex-boyfriend or -girlfriend in something like a cartoon. This is public airing of a grievances that is much better dealt with in private. It's also unwise to represent an old relationship because not only does it reopen old wounds, but it also may anger the person you used to date and create much wider, negative repercussions.
Making a cartoon that lambasts someone you used to date in a public and humiliating way is rude, irresponsible and inappropriate. Artists can make thier art personal, but they also have to respect boundaries and not slander or humiliate anyone in their personal lives. They need to be respectful, while remaining truthful.