Earlier there was a possibility to bring out the mirror from the iPhone just by opening the camera and then enabling the front facing mode but the only problem was this mirror was not instant.
The Instant Mirror application can be downloaded from Cydia only for $2.99 USD and considering the price of the app, many users won’t prefer to buy this app. But still then this app could be a boon for those who actually need it and also it has some additional settings which might be interesting.
Just search for it in Cydia to download.