To be fair to Mario they were lining him and Hughes on the same side of the formation for a lot of the day. This is part of what freed Hughes up with single teams because the Phins were committing two guys to Mario on the inside. I wouldn't expect Mario to have a lot of success at a DT and I would totally question the scheme if it wasn't for Hughes making big plays because of the single teams he saw.
The thing I'm mostly concerned about is that it appears they're using Dareus as a space eater instead of letting him penetrate and make plays. It's mind boggling why they'd do this as it's the type of thing you do when you have playmakers at LB and just an average big bodied guy at DT. It's the exact opposite for the Bills and they are not utilizing Dareus to the best of his abilities.
They were good in a different scheme, one where the DL was allowed to penetrate and disrupt the offense and all they had to do is come in and clean up. Now the D is filtering the plays towards them with the expectation that they'll read the play, fill the right hole - and actually make a tackle- and they are failing.