Why do you believe Daemon would’ve been one of the worst kings on the Iron Throne?

I don’t? 

As I said in my Blacks and Reds series, I think Daemon was someone who understood both the image and the substance of power.

He would have been a very different kind of king from Daeron. Here’s what I discussed last time:

more social mobility within the nobility. With the Lannisters, Tullys, Martells, Arryns, Baratheons, and possibly Tyrells removed as Lord Paramount and replaced, there’s definitely a lot more room to grow if you’re a knight with some chops vs. a world in which the same families have ruled the Seven Kingdoms for thousands of years. On the other hand, this probably means a higher level of social disorder and violence as those below seek to rise
redefinition of the monarchy vis-a-vis the Great Houses. Now here’s a tricky one. On the one hand, a monarchy that’s removed most of the Great Houses probably has a lot more authority vis-a-vis its chief vassals than, say, Aegon V had. Feudal monarchies work when the king has lands to give away and when vassals fear losing their fiefs. On the other hand, if you give people the idea that the Iron Throne’s dynasty can be replaced, you might be facing future rebellions from other dynasties.
Dorne divided. Given that the Martells are likely to be really unhappy about being unseated, it’s quite possible that Dorne leaves the Seven Kingdoms soon after joining them…but not all of Dorne. Instead, I see Daemon recognizing the Yronwoods as Princes of Dorne in control of the Red Mountains, given that that’s their natural powerbase, the stony Dornish are the least Rhoynish to begin with, etc. This is not good for Dorne, because it means that the Yronwoods and the Wyls and eventually the Blackmonts and the Daynes will be raiding south instead of north.  
Inheritance laws changed. Given the standard need for post-conquest legitimization (hence Aegon II’s decrees after the Dance), I imagine Daemon would change the laws of inheritance. Raising all bastards to the level of true-born sons would probably be too radical, but I could see something along the lines of making it easier for bastards to be legitimated, and giving legitimated bastards equal legal status, and making it easier for fathers to write wills choosing their heirs rather than going by straight primogeniture.

To elaborate and extend on those thoughts: 

• I think Daeron would have fought another war in Dorne, but instead of trying to rule through an imperial viceroy a la the Warden of the Sands, he would have raised the Yronwoods to the position of Lords Paramount of Dorne. (Not Princes) Distribute the lands of the Martells and their loyalists to the Yronwoods and their allies and divide and rule (possibly making use of the stony/sandy/salty division within Dorne). 
• In some ways, he would have more conservative than Daeron – certainly, his would not have been a court of women, poets, and Dornishmen, but rather a court dominated by the great knights who had fought for Daemon. On the other hand, his reign would have seen more social mobility within the nobility, as I said – with the Reynes as the Lords Paramount of the West, the Yronwoods as the Lords Paramount of Dorne, either the Freys or the Lothstons as the Lords Paramount of the Riverlands, possibly the Hightowers or Oakhearts or Peakes as Lords Paramount of the Reach, the Sisters raised to an independent kingdom under the Iron Throne, etc. The same thing would have happened on a local level as well as we see with Ser Eustace Osgrey. 

And there would have been a lot for him to deal with, depending on whether the Targaryens managed to hold onto a foothold in Dorne or escape to Essos. Certainly Daemon and his heirs would have had to deal with the Great Spring Sickness, the drought and the migrations, Dagon Greyjoy’s reaving, etc. 

Would he have been as good a king as Daemon? I don’t know. Certainly, if he survived the Spring Sickness, I definitely think he would have been better than Aerys I.