Smith likely called the Oracle "Mom" as a reference to the fact that she is the "mother" of the Matrix, rather than as an indication that she was his creator. There are several reasons why this is the case.
The first is that every program, including the Oracle, has one purpose. Her purpose was to investigate the human psyche, not write Agent programs to guard the Matrix. Agents and Agent-like programs have been necessary for all iterations of the Matrix, including the betas (which existed before the Oracle devised her choice-based model), so such guard programs would have been created before the Oracle influenced the design of the Matrix (the Agents' predecessors were probably created before even the Oracle herself).
Also, the Oracle's dialogue suggests that it was not her but rather the system (the Architect's "equation") that made Smith so powerful in order to counteract Neo's growing power:
[Smith] is you. Your opposite, your negative, the result of the equation trying to balance itself out.
The Matrix Revolutions (transcript)
I believe ultimately that she created Smith so Neo would have bargaining power with the machines when he goes to make the peace deal (beating Smith for the Machines in return for freeing humans / not killing humans).
The problem with this thinking is that the Oracle had no way of knowing that Neo would take the path he chose (which led to the end of the war) until after she would have needed to "create" Smith. She didn't know how it would all end even after Smith had assimilated most of the Matrix in the third film:
Neo: You helped me to get here, but my question is why? Where does this go? Where does it end?
Oracle: I don't know.
Neo: You don't know or you won't tell me?
Oracle: I told you before. No one can see beyond a choice they don't understand, and I mean no one.
The Matrix Revolutions (transcript)
The virus Smith was created at the end of the first film (when Neo destroyed Agent Smith). At that time, Neo didn't even know which door he would choose when he visited the Architect (the door to the Source or back to the Matrix) much less why he would make that choice (to return to the Matrix to save Trinity, as it turns out). Since Neo didn't understand that choice, he couldn't see past it -- which means the Oracle couldn't, either. Neither Neo nor the Oracle could have known that Neo would ultimately strike a deal with the machines to destroy Smith in exchange for saving Zion at the time that Smith became a virus. If the Oracle could not see that far into the future then she'd have no reason to create Smith.
What happened is that the Oracle saw that (1) Smith had become a virus and would become powerful enough to destroy the Matrix and (2) Neo had a "profound attachment" to Trinity in particular which would cause him to return to the Matrix rather than go to the Source and start the next Matrix cycle. Taken together, these two pieces of information mean that the Matrix cycles will not continue: either Smith would destroy the Matrix, or the Matrix would collapse due to Neo's failure to return to the Source to start the next cycle. She saw an opportunity to end the war between the humans and machines by uniting them against Smith, and she took a gamble in guiding Neo down that path. The key, however, is that she took this gamble after Smith became a virus. She was adjusting her plans as events occurred, rather than engineering them in advance.
But what motivation would the Oracle have to do this if she is just another program in the Matrix used to pacify humans.
Although the Oracle did not create Smith herself, this is still an interesting question. Why would the Oracle -- a machine program -- work to end the Matrix system she helped create? Her answer:
Oracle: I want the same thing you want, Neo. And I am willing to go as far as you are to get it.
Neo: The end of the war.
[The Oracle nods]
The Matrix Revolutions (transcript)
The Oracle realized that the humans and machines could co-exist without war if (1) the machines were willing to give up some control over humans by letting them exit the Matrix if they wanted to and (2) the humans were willing to allow some humans to choose to live in the Matrix instead of sacrificing all the blue pills in their quest to the destroy the Matrix at all costs. She did all this in order to end the war, which had the potential to completely eradicate both the humans and machines.