To the extent that the "We don't have a star refrain" has been based in a feeling that the Celts lack that essential element of a good team: the high usage scorer you can depend on for 20+ points night after night, then yes, we can put that to bed, I think.
Isaiah Thomas is an exceptional scorer. Play him 30+ minutes a game and he'll put up 20+ points more often than he doesn't, even if there aren't any other really potent scorers on the floor with him. Take it to the bank.
I guess there are lots of definitions for what a "star" is, though. I think Isaiah qualifies, at least in many senses of the term, because he's the unquestioned offensive leader and primary scorer for a team that's on track to make the playoffs. He's a pretty good playmaker, too, and he competes to make plays for rebounds and on defense, so he's not just a guy who gets you points.
While I don't know where Isaiah or Crowder properly rank in the hierarchy of NBA players -- they're certainly better in the roles they currently occupy than I expected they could be -- I do think they represent a good starting point for the Celtics. This isn't a great team by any stretch, but with one or two significant additions it might be.
What the Celts unquestionably have right now is something they didn't have a year ago: at least one player who is regularly, on his own, worth the price of admission. It's really fun to have a player like IT on the team, and he perfectly embodies where the Celts are at right now: scrappy, undersized, frequently underestimated, never satisfied, probably not talented enough to go as far as he'd like to go, but persistent and impressive nonetheless.