I pretty much gave up on Mint's net worth calculations, partially because of the Zestimate and partiaully because I never connected our 401k accounts to it.

I actually keep a spreadsheet with the breakdown of each payment (principal, interest, escrow), remaining balance, and my (very conservative) notion of the house value. Takes a few minutes to update it each month, but I kind of like doing it. Gives me a chance to notice that each month we pay off an extra buck or two of principal, pay an equivalent amount less interest, etc. And I can easily look at where we were a year ago. Plus knowing the P&I number is useful when considering whether to pay off the mortgage, since that's the only part that would actually go away.