So the plot of Friends Forever Issue 38, while frankly unremarkable, did introduce an interesting idea: that Celestia previously watched over the dreams of the ponies of Equestria before Luna took up the role upon her return. In this episode, however, Luna indicates that Celestia has no power in the realm of dreams. Now, realistically the comic writers probably forgot about that or ignored it for the sake of their story, but a couple alternative suggestions do present themselves:
1. Luna didn't mean that Celestia had "no" power, just that it was so little compared to her own that Celestia wouldn't have been able to help; the only way this answer would make much sense would be if dream-tapping is just so counter to Celestia's natural magic that she can only do so much.
2. Luna was lying because she didn't want Celestia to be involved anymore than she wanted to involve Twilight and the others. Luna's whole guilt trip certainly lends itself to this explanation-she was pretty much forced to call upon Twilight and the others for help, and involving Celestia "unnecessarily" would undoubtedly have only made her feel worse.