How to Get Your Tumblr to Have an iPhone Versionby Aaron Parson
The iPhone can view full websites in Safari, so mobile visitors to your Tumblr can see your page the same as on a desktop computer. Many sites, however, opt to display simplified themes on the iPhone in order to better fit the small screen and use less data. On Tumblr, you can pick whether or not to use an iPhone theme version in your theme customization settings.
Enabling the Mobile Layout
To turn on the mobile layout, open your blog's settings and click "Customize." Scroll down in the left frame and click "Advanced options." Click the slider next to "Use optimized mobile layout" and click "Save." Tumblr automatically enabled this setting when it first added the option, so you should only need to turn it on manually if you or your theme disabled it.
Custom Mobile Layouts
Some custom themes include code for iPhones and will override the default mobile layout. However, if you use a theme that already looks good on an iPhone but does not use a separate mobile layout, you should not turn on "Use optimized mobile layout," as doing so will override your theme with Tumblr's default mobile theme.