According to the Apple press release from January 2013:
The Company sold a record 47.8 million iPhones in the quarter, compared to 37 million in the year-ago quarter. Apple also sold a record 22.9 million iPads during the quarter, compared to 15.4 million in the year-ago quarter.
So, on average, Apple sales twice as many iPhones compared to iPads.
Also, the first iPhone was launched in 2007 while the first iPad came on the market in 2010. So we can assume that there are at least 4 times more iPhones than iPads.
People tend to carry their iPhones everywhere (even in some places they shouldn't) so the chances that somebody would find and use your app on this device is way bigger.
And, last but not least, most of the apps on iPhone work on iPod touch as well.
First, of course, you have to decide is the app can bring value to the customers having an iPhone.
If it can, then the answer is definitely "Yes, you should create an iPhone version as well!".