That's definitely a big part of it. It's not so much that Morgase isn't very intelligent but for a long time she in effect co-ruled with an hidden grey eminence, her reputation growing around. Not that it's the mark of a bad ruler to surround herself with competent advisors, of course.

First, it was Thom Merrilin who more or less won her, a sixteen years old who was supposed to only rise to High Seat and was probably trained for that job only, the Succession to the throne. When it became obvious she would win, Elaida entered the fray with the power and influence of Tar Valon behind her. Morgase had those two at her side to navigate the dangerous "Taringail years", beside having the strong traditional allies of her House behind her. Then Tom killed Taringail, Elaida seemingly got him embroiled in a plot with his nephew and he ended up ousted from Morgase's side, but then she had the cool-headed Bryne and Elaida who can also be cold, cerebral and calculating when her pet obsessions and ambitions do not get in the way. As far as we know, except for alarming bouts where she let her emotions get the better of her (like with Thom), Morgase was a competent enough, well-educated, well-surrounded monarch.

The first big political mistake we hear of (aside from ousting Thom) was to have failed to rein in Elaida when she totally misread the political climate in Andor and insisted that Morgase organized the whole "Logain show" in Caemlyn, against her better judgement. Elaida had let her obsessions get the better of her, and put the interests of TV ahead of the nation she served as advisor. Pedron Niall had won this one, sending WC agitators way ahead of Logain's convoys, utilizing the growing discontent about the shortages due to the weather to full extent. If not for the fact the Shadow had its own reasons to restore order in Andor, it's hard to tell how weakened Morgase's rule would have ended up. Of course without Rhavin she would most likely not have antagonized all her staunchest allies among the Great Houses nor raised to prominence her most dangerous opponents who were waiting to backstab her.. so maybe after a few months this would have abated.. but with Rand conquering around... maybe not either.

After her escape from Andor, devoid of advisors beside Lini, it was often one bad decision after another, though some of them indirectly resulted from Compulsion (her clouded judgement about TV leading her to conclude her only friends could be the WC... That didn't go so well, and she realized it fairly fast, but too late).