Safari for iOS doesn't have a built in way of showing html source of web pages. Luckily there are apps that can help.
I have chosen View Source by Paul Hudson. With it's action extension it is easy to show the source code of web pages. You can choose between a bunch of different color schemes. Solarized light and solarized dark are my favorites.
In addition to show and copy the code View Source can list all assets of the page so you can find images, scripts and links quickly.
If you don't like action extensions you can feed the app a URL directly and view the source code that way.
View Source is 1 USD in the App Store and works for both iPad and iPhone.