People with much more strategic and tactical knowledge than any of us have debated this issue ever since 1944, which makes this a daunting question to answer. In my opinion, however, the plan was flawed because it was simply too daring and, more importantly, depended on too many things going right. Military people often say that simple plans are the best because they have fewer "moving parts" that can go wrong. Operation Market Garden was not simple and some of its moving parts, not surprisingly, went badly wrong.
One of the biggest problems with the plan is that all of the parts had to work in order for the plan as a whole to work. If any part did not work (particularly if any bridge was not taken) the whole plan would fall apart. It seems reckless and overly optimistic to attempt a complicated plan when the failure of any part of that plan will doom the entire operation.
Another reason the plan was flawed was the fact that the ground forces' supply lines were not very good at all. Many harbors that had been counted on as entry points for supplies had yet to be captured or, like Antwerp, had been captured but were still cut off from the rest of the Allied forces by German troops. These logisitical problems made it very difficult to get the ground forces the supplies they needed.
Finally, the plan was conceived and executed under political pressure of a sort. There was a constituency in the Army that believed in the value of Airborne troops and were chafing to get Airborne involved. There had been no major airborne assaults and these people pushed Eisenhower to launch Market Garden in part to prove the importance of airborne troops.
So, to me, this was a case of plan that was overly optimistic and which was approved, to some extent, for political reasons. I think that it was not a very good plan to begin with.