No, you must sponsor your wife's citizenship by filing the required papers with the U.S. Citizenship and Information Service(USCIS). You can consult their website for further information on the process involved.
For over 25 years, there has been no requirement for US citizens to "sponsor" the naturalization (citizenship) applications of aliens. Before that, there was a requirement that at least 2 US citizens sign off on the naturalization application of an alien (much like the current requirement of the various bars that 2 currently licensed attorney support the application for admission of a bar applicant).
A naturalization applicant is approved on her own merits. If your wife/fiancee is a legal permanent resident, she does not need your support or approval to apply for US citizenship if she qualifies for naturalization.
Under the current laws, people over 18 do not automatically gain US citizenship under any scenario. Marriage to a US citizen does not automatically grant an alien any immigration right. If an alien is entitled to some immigration right by marriage, the appropriate application/petition needs to be filed and approved.
To be eligible for filing the application for naturalization, the alien must generally first be a legal permanent resident. Some aliens may qualify for legal permanent residency because of their relationships with US citizens. If your fiancee/wife has no legal immigration status, she may be eligible to gain legal immigration status if an appropriate application/petition is filed for her and is approved. There are many reasons why an application/petition will not be approved.
Immigration laws can be complex and unforgiving. If an applicant does not meet all the statutory requirements or submit all the required documentation, that is a basis for an immigration application to be denied. You and your fiancee likely should review the facts and options with an attorney.
If you want to look over the forms and some information, they are available for free at the government's website at www.uscis.gov .
Congratulations on your engagement!
When you get married, assuming that is in the United States, see an immigration attorney who will file a petition for adjustment of status to change your wife's status to lawful permanent resident.