Probably not a good sign for Biden, if both of his two main competitors in the moderate lane get a post-debate boost in polls.
I feel the opposite, if moderate candidates (and Buttigieg is not moderate, he is just smart) start getting more positive attention and it becomes more than just Biden vs. Everybody, then we can start having a real debate, and moderates will win because they are right.
In terms of horse-race numbers, this would mean Biden might go down a little, but Biden+Buttigieg+Klobuchar bloc goes up. Since Butti and Klobb aren't going to be around past Super Tuesday or so, and are likely to give their delegates to Biden at the convention, it's a win for Biden when all's said and done. Although it might make it hard for him to win Iowa.