I'm A Banana
Potassium-Rich
What I don't like about this is that it doesn't properly reward creative solutions that make what are designed to be hard fights much easier. It encourages players not to play at their best because that makes the fight "harder" and any player who isn't completely oblivious to the fact that the DM is making up how much XP they get will figure out that those numbers go up when they have a "harder" time.
If the fight is absolutely trivialized by the party, the best thing the DM can do is come prepared next time for a more proper challenge. Otherwise I would reward my players more for creative thinking that made a fight easier than for not doing so and making the fight harder than it needed to be.
Of the many masters that XP may serve in a game, I personally find that using it as a reward for good play is one of the weakest. In large part, this is because it's hard to see any real change in dynamics because of more XP.
Like, if I give out XP for good play as a reward, that ultimately causes the party to gain levels faster, which in effect just causes them to fight stronger monsters. "You did really good on that goblin encounter, so you're going to be rewarded by fighting hobgoblins now!" doesn't feel like much of an actual reward, even if the "big number getting bigger" and "new level hotness" aspect of handing out XP does feel nice.
What I tend to favor - and what 5e makes easy to favor - is treasure as a reward for good play. Magic items and GP. This stuff enhances the performance of a character across the 2/3 pillars of play (GP is mostly an RP resource, magic items are mostly a combat resource). GP can have that "big number getting bigger" phenomenon, and is fun to find ways to blow. Magic items are a bit of randomized new level hotness.
So rather than give them more XP, I'll give them more treasure - more GP, more magic items. If you take down a deadly enemy with smart play, you don't earn extra XP from it, but you DO get the thing's full treasure hoard, and if you didn't use many resources, you can go to the next source of goodies almost instantly!
YMMV, of course, I just thought it was worth pointing out that if you're looking to reward smart players and good play, it might be worth looking to places other than XP for that anyway, since XP doesn't always feel like a reward.