Drew Barrymore is weighing in on folks who are now backtracking/apologizing for joking about Kate Middleton before they knew she had cancer ... including Stephen Colbert.

The actress/talk show host spoke on the phenomenon that's been unfolding over the past week or so -- namely, the fact that people are getting dragged for having made light of Kate's mysterious situation ahead of her cancer diagnosis announcement.

There've been a lot of people who've had to eat crow over this -- and that goes for Colbert too, who dedicated a little time on his show Monday to express regret for his recent bit.

As we reported ... SC leaned into affair rumors on one of his recent late-night episodes, poking fun at the possibility (or the gossip, more like) that some suspected Prince William of being in an affair with Lady Rose Hanbury ... this while speculating on Kate's whereabouts.

Colbert was far from the only one who cracked jokes about it all -- remember, Blake Lively has apologized for her own quips about the whole thing before it was clear how serious it really was -- but on the issue of demanding retribution against these folks ... Drew's eye-rolling.

Check out what she had to say about those who's clutching their pearls on behalf of the Princess -- as she wisely says ... we all need to show a little patience, and a lot of grace.