I recently discovered the "Tag" function and use it for this purpose. I've set the name of one of the tags to debt principal. Anything that is 100% principal (0% interest or an extra payment) will get tagged in the "edit details" function under the transactions tab. Anything that's P&I is split into 2 amounts, and the principal portion is called "debt principal" or something I made up in the financial section, plus it gets tagged.
This sets the table for getting the reports you are asking about, but there's still one more step. When you're in the "trends" section on the left side there's a section called preferences. The default setting is No Hidden Tag. Click Edit, scroll to the bottom, and click the button to hide your designated tag from budgets and trends. Viola, the debt principal disappears until you "un-hide" your tag.
I know this is not a simple process, but it's important enough to me to take the couple of minutes to set up and track moving forward. I want to see expenses, not cash outflow. I realize the purchase of the house, car, education, or whatever is an expense and Mints' tracking process really captures your true cost, but I don't like it. I'm using Mint to track my current spending, not prior purchases.
If anyone has a better way let me know.