The DM is more then a calculator. The DM shouldn't be a computer program mindlessly following the rules. The DM should ignore, break, bend, change, and make up the rules as needed to present the game.
The DM modified the Ogre based on the fluff. Good for him, that's good DMing.
No, no it isn't. Improvisation and dynamically adjusting things on the fly
can absolutely be a good thing, but is certainly not automatically so.
Now, this is definitely something that will vary from one group to the next. But for many groups, the point of having a shared rules system is to ensure the players can have reasonable expectations about how the rules work and how things happen... rather than the have the DM arbitrarily change things on the fly, often to the detriment of the game.
Now, I've seen groups where DMs do fudge dice and alter stuff. Maybe with the goal of making things more cinematic. Maybe because CR ratings or Encounter Levels or the like aren't reliable, and the DM likes to finetune things as the combat goes on to ensure the PCs have a 'proper' challenge - one where they triumph, but feel the difficulty of the fight before doing so.
Some groups like that, some don't. But either way, it is definitely a rather advanced form of DMing and requires a pretty solid ability to make those adjustments without PCs feeling like the DM is just deciding what happens without them having any input. As you mention, maybe the player could ask for a save against the effect, or argue for the chance to use Acrobatics or the like. Maybe the DM will allow that - but, in the end, it is still up to the DM. When actually playing by the rules, the players can at least appeal to those - but once you toss those out of the picture, they lose all agency in the matter. The DM decides the player should go flying, then decides the player should land in an acrobatic pose.
It might make a neat scene, but odds are for the player, it stops feeling like them being awesome, and more like they are just part of a puppet show being put on by the DM.
Which is, ultimately, what you are advocating - the DM as storyteller, and the players as an audience, rather than as participants.
And, honestly, I say this as someone who has done exactly this sort of thing at times. Sometimes to the player's advantage, sometimes to their disadvantage. But it is always something you need to be careful about. And anytime your 'cool fluff idea' results in arbitrarily knocking out a PC without them getting any way to respond? That is probably right at the top of the 'bad DMing' chart.
Finally, how do you know the Ogre didn't have a mighty smash ability that knocked targets back?
Well, based on the description of events, I'm guessing the DM said something like, "Hey, guess what would be cool - the force of the blow hurls you backwards and into the wall! I guess that does (roll random dice) xx damage, knocking you out.