I don't think so. Seguin just scored again to complete the hat trick and make it a 5 point night. That puts him up to +14 on the season, which trails only linemate Jamie Benn (+15) and Valeri Nichushkin (+17), who spent a lot of time on their line as well.
The thing is, Dallas had an even goal differential on the season coming into the game. They're not a great team and that's going to reflect in the +/- numbers. For comparison, the Bruins are second best in the NHL at +57 goals as a team. They're, on the whole, going to have strong numbers across the board. Looking quickly, Seguin appears to have excellent Corsi numbers, a great penalty differential, tough quality of competition, and a very good +/- for his team. None of that is terribly shocking if you've seen Seguin play though.
It doesn't show or say that Seguin has been a defensive stud, but he's at minimum more than making up for it with his offense. Guys like Garbutt, Horcoff, Fiddler, and Peverley are on the ice for for fewer 5-on-5 goals/60 perhaps indicating they're better defensive stoppers. But they score so much less that a lot of them have negative differentials. Benn and Seguin, while on the ice for more goals against, are still each over +1 goal per 60 minutes of 5-on-5 due to extremely high scoring rates. Jamie Benn is 4th in the NHL in 5-on-5 GF/60 and Seguin is 19th. (And these stats are before Seguin's 5 point night is factored in.) They are flat out producing.