Tumblr is a great blog hosting service with such a clean interface, easy to use, free from any advertising. But when it comes to tumblr blog if you are viewing them on your mobile phone they should be optimised for proper viewing without any issues. Tumblr itself has an option for any blog which makes any tumblr blog optimised to be loaded on mobile phone.
By default this option is checked, but if in your case it disabled then you can still enable it by following procedure given below.
1. Go to tumblr.com and login to you tumblr blog dashboard and then click the link in the right sidebar Customize as shown in the image below.
2. On the horizontal customization bar, click Advanced you will see the option which you need to check to make your blog compatible for mobile phones like iphone, blackberry and other smart phones etc
3. Click Save + Close once you are done configuring this option.