The series does not explain the meaning of the letter 'i' in the series iZombie.
Does it have a meaning?
As far as I can tell, there's no "official" explanation.
The title comes from the comic book it's based on iZombie by Chris Roberson and Michael Allred.
There are some conjectures on it, though.
On reddit, someone guesses that it could be read "I, Zombie"... as a personal account of someone who has become a zombie or an homage to other works of fiction.
- She's a zombie, she also monologues a lot. So the title is basically her saying that she's a zombie, while also trying to be catchy.
- Perhaps it's like a homage to "I, Robot", but also trying to be trendy?
Why not name it that directly... well, there's already a movie titled I, Zombie, so using it for the comic would be confusing.
Also, we're in a day when Apple popularized the iSomething format, so it's not as if we haven't seen it before.
The statement of "I, [...]", wherein ones name can be inserted, is traditionally associated with an oath or statement of truth.
This can be a legal document (such as a will or testimony), or more broadly a solemn statement of truth:
A statement of truth states that a party believes the facts stated in a document to be true and accurate. It should be signed either by the party or, in the case of a witness statement, by the maker of the statement.
It is a verbal signatory that the following statement or story is, in the speakers intention, fully true and honest.
In the case of iZombie, it is an assertion of Liv Moore's version of events.
"I" in front of any name of any creature or creation is a statement of being self-aware and be able to make calculated decisions. Normal zombie movies or tv shows have zombies running or walking around in autopilot mode. Another movie example would be I, Robot, where robots become self-aware and go against their programming.