Like Discord, Celestia is essentially the same. As another immortal being, a few decades is a very short amount of time.
 
One of the biggest differences, aside from motherhood, is that Celestia has lightened up quite a bit. She’s a lot less serious, and lets out her more fun side more often. Celestia has good relationships with both of her children and Discord.
 
When Celestia became pregnant, her relationship with Discord had been a secret. Both of them feared what the public would think, but soon after her pregnancy she proposed (as if Discord would ever have the guts to do it). The two did receive some backlash, but most of Celestia’s subjects remained loyal, and trusted her judgement.
 
Celestia and Luna are very close, arguably closer than they’ve ever been. Celestia also still helps Twilight with making big decisions from time to time, but at this point Twilight is very confident and capable, barely needing guidance to perform her duties. Celestia has also branched out, and has allied with many new outside nations, earning their trust and keeping the peace.