There is good news and bad news. The bad news first..there is no way within the iPad's native Mail app to format mail - so you can't change font, underline, bold etc.
Now the good news - the iPad's native Mail app can display HTML formatted mail. So, if you prepare your message using one of the many HTML-editor apps and then copy and paste it into the message area when you want to send a mail it will be formatted accordingly. It's a bit of a workaround, for sure, but it does enable you to send formatted mail (including embedded images, fonts, font styles etc) from a mail sent with the iPad's native Mail app.
Tim