I imagine a lot of teams would categorize maintaining the status quo as a good off-season. If your expectations were to go above that, then I guess the Celtics did have a disappointing summer.
As I indicated above, I think there are plenty of teams for whom maintaining the status quo could be a successful off-season. If you've already got a contender on your hands, or a talented young team with all the right pieces in place already, that's fine. For a team still searching for a long term formula, I don't think it's enough.
That doesn't mean it was a disastrous off-season that is going to cripple the franchise for years or anything like that, but outside of Dallas and Portland, I don't think there are many teams who got totally screwed this summer.
Here are some other candidates, though:
I might point to Sacramento, who made a questionable pick at #6 and a horrible trade with Philly, but they retained Cousins without firing their coach and have an outside shot of being a 35-40 win team this year, which would be a major victory for that franchise, sad as that is.
Toronto is another candidate for me -- two years in a row they've gotten exposed in the playoffs and yet they mostly stood pat this summer, aside from committing big time to a guy who was probably a product of a quality system and great coach in Atlanta.
I think Chicago moving on from Thibs counts as a disappointment for them, even if everybody knew it was coming. Committing to an unproven head coach without making any major roster changes is a risk. I think if I were a Bulls fan I would have wanted to see them be more bold, because that roster peaked a couple years ago.
Denver was a mixed bag; they got Mudiay, who probably fell too far in the draft, but they had to move on from Lawson without getting any real value in return, and they committed long term to Wilson Chandler and Dano Gallinari instead of moving them for assets, which is a major headscratcher to me.