So what do you think petting Celestia's mane would feel like? Do you think Celestia would like pets if she considered you a good friend/lover? Of course, you ask for permission beforehand, and in this scenario, she says yes.

How would Celestia react to receiving tender petting from you?

Or even if she considered you a good friend, but not quite a lover yet, would loving pets warm her up to considering you as a lover to some degree at least? Probably not fully, you may need to do a far more romantic gesture.

I believe Celestia would very much like being petted. Considering how stressful she must be with ruling a country and the nightmare called "Day Court", getting affection in the forms of pettings, scratches and cuddles must be heaven for her.
I've kinda always imagined Celestia's mane having a sensation similar to long silky hair dipped in luke warm liquid, not necessarly the wet sensation, but more the overall smoothness.