For now html code will be posted as plain text.
I think it would be great if you done that feature.
Thank you for the new feature request, we value every idea and we have a lot of ideas on our to-do list. We add everything there and evaluate which ideas are most requested by our users and implement those first.
Actually you can post html to tumblr, you just need to go into your tumbr account and enable html editor somewhere in the settings, when mp will post html tumblr will be able to interpret it and post as html.