“- Is a man not entitled to the sweat of his brow?

No, says the man in Washington; it belongs to the poor.
No, says the man in the Vatican; it belongs to God.
No, says the man in Moscow; it belongs to everyone.

I rejected those answers. Instead, I chose something
different. I chose the impossible. -”

Similar situations with Nicodemus against Arthur, or Arthur’s plan to bring adults from Earth into Dreamland. Either one is at the point of no return.

However, you are making some unfounded assumptions. The first, and most glaring (to me) is the assumption that magic is easy. If magic were easy, everybody would be using it, as is indeed the case in Dreamland. However, it is possible that in the “real world”, magic is extremely difficult to master, requiring a lifetime of study by a unique mind such as that of Merlin; thus magic could be a powerful but limited force, capable of amazing feats but no barrier to the developments of technology for more widespread advances to convenience, social equality, etc.
Your second, and perhaps more subtle but no less important false assumption, is that some measure of equality and an improvement in the quality of life could not be achieved in Camelot without the use of magic. I don’t believe this to be accurate. Granted, it’s extremely unlikely that peasants in Camelot had the same rights as knights or kings; Camelot is not a democracy in any version of the Arthurian legend. The Round Table was only a small step in that direction, representing a step toward equality between Arthur and his knights; no peasants sat at this table (except perhaps to clean it between uses). That said, wise policies on Arthur’s part, such as minimal taxation as required to maintain adequate defense and infrastructure, and rule of law rather than royal or noble whim, would go a long way to making a peasant’s lot in Camelot a good deal more comfortable than anywhere else in the area. The application of such policies might be aided on occasion by Merlin’s magic, but would in no way be dependent on the widespread availability of magic.

So it’s entirely reasonable (logical, even!) that Arthur, for all his naivete and idealism, could very well have been an effective an popular ruler. Sure, the castle of Camelot was built by peasant labor. But perhaps they were well fed and protected during this labor, as they may not have been in neighboring kingdoms. Nowadays a leader who creates jobs on that kind of massive scale is considered a hero …

Finally, returning to the original point in your first few sentences, you say that “I haven’t seen any evidence of Merlin casting any magic. Nor any evidence that the peasantry are not being oppressed.” You want logic in the story; here’s a logical truism: Absence of evidence is NOT evidence of absence.

(Congratulations, you’ve dragged me further into this philosophical debate than anyone else has!)

On the plus side, you can always go back and change it easily enough at a later date.

So there are two possibilities:
a. Merlin and/or some other magicians are using magic to allow Camelot to have all the food and resources it needs so that all who live there are equal and happy. The upshot of this is that people are after Camelot either because everybody there has everything they need, in which case it’s not the building or the idea they’re after, but magic. The conclusion one can draw from this is that Merlin is not very nice, keeping all the magic to Camelot rather than teaching others how to use it to improve their lives, and that Arthur is lying saying his land is ‘an ideal’ rather than telling the truth.
b. The past of the ‘real world’ of the comic had magic, used widely to make the lives of peasants all over medieval England (or at least whatever Arthur ruled of it) much happier. Nobody in the present remembers this, it was in no way recorded or suggested by archaeology and people went on to develop technology to do what magic was doing anyway. In this scenario, as stated earlier, Arthur is a bit odd for being obsessed with Dreamland where there is magic that means everybody has what they want and can live in peace, because apparently his own kingdom already has this, which can only lead to conjecture as to why there’s so much dissatisfaction that people will attack the King in small roving bands insufficiently equipped to take on armoured knights because they’re that angry.
c. There is not magic, or at least, no more than simple spells for divining etc and Merlin is mostly just a wise man with an interest in alchemy. This would mean that Camelot was built and is tended by the hard labour of hundreds, maybe thousands of peasants while Arthur dreams his days away. Arthur’s idea that they ‘want Camelot’ due to simple greed shows that he’s completely out of touch with them and is really not a very nice or effective king.

So, yeah, under any scenario I can think of, Arthur is not a nice guy or an effective King. I know it’s sort of a kids comic, but I expect there to be logic in a story, even one with magic and aimed at children.