I think they produced too much too fast. I think they expected to make tons of money, but some of the properties they had (Lucy and esp. Married with Children) weren't lines Mego collectors wanted. Lucy collectors have the nicer Mattel dolls to collect, so the ugly 8" ones didn't appeal to them much, I'd guess. I have some of their bodies for customs, and they are as bad as everyone says they are. They did a decent job with the Mad Monsters, as I do enjoy my set of those. The Munsters seem well made. If they'd stuck with this kind of quality and fun TV lines with more broad genre appeal, I think they would have done well.
Overall, I'd say they did well at first, but the glut of merchandise, and the lack of quality probably made the line a failure over all.