If one interprets this question as Why were the Merovingians so reviled at the peak of their power?, then the answer is easy: they weren't. At the peak of their power, the frankish kingdoms were the most powerful geopolitical entities in Western Europe, were recognized as such and their kings were treated accordingly.
The early Carolingians reviled the Merovingians in order to get the Pope to legitimize their dynasty as kings, and not just rulers, of the frankish kingdoms. This finally happened when Pope Zachary deposed Childeric III in 751 in what can only be described as a very good operation for both: Pépin gained the legitimacy he was after while Zachary ensured the domination of the Catholic Church on the frankish kingdoms (while the Franks had been Christian for several centuries at that point, the frankish bishops of the 8th century had a reputation of indiscipline bordering on heresy, see for instance Aldebert, as well as of venality and the frankish lords were quite tempted to confiscate Church land when they needed it, see for instance the matter brought to Zachary's attention by Boniface before and during the Concilium Germanicum). Besides, he gained a powerful ally able to counter the military threat from the Lombards he was under.
Finally, French republican historiography (see for instance Michelet) liked to go at absurd length to make fun of the late Merovingians, in part because as kings without kingdoms they had in the view of the XIXth century French Republicans all the vices of the kings and none of their virtues (the embodiment of the French Nation), in part in repetition of Carolingian propaganda, in part because of the contingent fact that Dagobert the First (in fact one of the most powerful king of Western Europe in the seventh century) was said to have been quite clumsy, as was Louis XVI, and hence became the hero of a very popular song during the French Revolution (the song's popularity, as a children rhyme, endures to this day, so that French children know to mock Merovingians well before they know what a Merovingian is, or in fact what a kingdom is).