Recently, FiveThirtyEight released its projected wins for each NFL team, and it projected that the Niners would finish last in their division (granted one of the toughest divisions in football) with 6.6 wins and a probability making the playoffs of 12.4% ("2015 NFL Preview: Everything You Need To Know About The Seahawks, Cardinals, Rams And 49ers"). FiveThirtyEight has a fairly decent track record in this regard. Using a similar approach it correctly projected that the Golden State Warriors would lead the NBA in wins and had the highest probability of winning the championship. Thus, we should take its rankings seriously.
Of course, key injuries to the Niners or other teams in their division could dramatically shift the odds, but the 6.6 wins and 12.4% playoff probability can serve as a benchmark by how we can judge the quality of Niner coaching. In other words, if the Niners finish with 6 or 7 wins, then the coaches didn't do any worse or better than what would have been expected with the talent they had to work with. However, if they win 10 games or more, then Coach Tomsula should be seriously considered for coach of the year. Conversely, if they only win 2 or 3 games, then the Niners may be in need of another coach (but I doubt that either Jed York or Trent Baalke would admit they made a mistake after one year).