According to FootballOutsiders' stats, Seattle's offense is currently 19th in the league in DVOA and 18th in weighted offense (which puts more emphasis on the most recent games as the season goes on):
http://www.footballoutsiders.com/stats/teamoff
They're 14th in yards per game and 22nd in points per game:
http://www.espn.com/nfl/statistics/team/_/stat/total
"Atrocious" is probably too strong of a word in retrospect, but that level of production is extremely disappointing for an offense stacked with talent like Jimmy Graham, Team Leader Rawls, and Doug Baldwin - not to mention a supposedly "elite" "franchise" quarterback (according to his slurpers).
So why are they still winning? The average NFL team in 2016 scores about 23 points per game. Seattle's defense has held their opponents below that number for 9 of the 15 games they've played. Even in games where Struggle and the offense have completely the bed, their defense is so mega-stacked that they still have a chance to win more often than not (see: the win over Miami in Week 1, the tie with Arizona in Week 7, and the razor-thin losses to the Rams and Bucs).