They got better.
Sunset actually evolved past the obsessive power fixation that literally demonized her in the first movie and they made amends after Sunset actually achieved the emotional maturity Celestia was hoping for her in the first place, shown in one of the later movies.
Unlike Renegade's Sunset, who as far as I can recall as done absolutely nothing to show she isn't one bad day from literal demonization.
The thing is in canon we don't get to see the relationship they had- the progression from loyal student to futitive- plus canon sunset basically lost her magic when she went to the human world, which probably made all of her issues worse.
But Celestia has mad a number of questionable decisions, which various fics have called her out on- sending Twilight and friends to save the crystal empire and calling it a test instead of having her or luna go to fight the most dangerous non alicorn pony ever, Getting fooled by Chrysalis' bad acting when after Shining armor who was brainwashed and Twilight who was suspicious. Celetia is a person with flaws and the main one is that she is so competant and smart that combined with litterally having more experience than any other pony, she is used to being right and treated as always being correct.
Basically between her age, talent, and position, noone calls her out on things, even Luna didn't- instead turning into Nightmare Moon and fighting her. Everything that lead to Sunset going rogue was Celestia's fault, its basically the same as Tai Lung and Shifu in kung fu panda, except Shifu wanted Tai lung to be the dragon warrior and messed up his head by pushing him until when Tai Lung was denied he lashed out. In Celestia's case it was more like she was a combo of Shifu and Oogway- training Sunset but keeping some things away with no explaination. Sunset went with it until she couldn't, buying into the whole always perfect image of Celestia, until given the chance for something similar, being told no, and never getting a real explaination, painting every past denial in the same light.
Sunset wasn't wrong to want what she wanted, and even canon shows that it is possible to become an alicorn, if not easily but in canon Celestia failed to get her worries across to her, and here the disfunctions of the relationship are made clear- Celestia had nothing to do with Sunset's emotional maturation. Though to be fair par of this is my bias because Twilight's own instability and how Celestia often feeds into it doesn't paint a good picture for her ability to guide and help people that way- as does the older case of her sister becoming an insane supervillian right under her nose, or even after they are back and together a late episode shows they still have a bad relationship of constantly misunderstanding and not seeming to appreciate each other.
This Sunset got the support and help she wanted, became an alicorn, and has done nothing wrong- outside the questionable activities Gravyen occasionally gets her into- and unlike with celestia she actually made friends because Grayven found people she could be peers with rather than the sort of telling her to get out of the lab and passive agressive prodding Celestia did with her
Actually, the argument can be made that Darkseid is the God of Evil, not Tyranny. If Highfather represents all that is good, then Darkseid is the flipside (flipseid?) of that coin. This belief makes a lot more sense to me, and explains why Darkseid is so powerful. After all, "Darkseid is."
Now, I've never watched MLP, so I was basing my bet on some assumptions. I'll amend it to say that the Pony Force (you know, Celestia with her Elements friends, and probably the other ones, too) can take him.
The video was long so I skipped around- but my take on it is that darkside and Highfather are the gods/embodiments of particular flavors of good and evil- Tyrany and Fatherly care- basically Good and Evil in positions of authority and leadership. Darkside is the best thing to Pure Evil in DC comics because he is evil that seeks to impose itself on everything- though with his pursuit of anti-life and his empowerment witih the Omega Force I could buy him growing beyond Tyranny into more general evil, but he is the new god of tyrany.
For the purposes of the original discussion, he probably stomps Celestia unless the nature of equestria nerfs him somehow- not impossible with the tree of harmony and how it may be tied to Equestria. If nothing else the Elements would probably be like holy water to a vampire for him.