She seems to understand that strength in the One Power does not mean the same thing as channeling capability, so while she might be weaker than Semirhage, she may have been taking her personality and skills into account. As we've seen, while Semirhage certainly has courage when necessary, she's not the smartest Forsaken though she is by no means a dullard. Basically, Semirhage can be easily distracted whereas Mesaana can be focused, and it is just interesting how both personality types have their detriments and advantages.

in some ways, I find her similar to Moghedien - because they don't have exceptional strength in the One Power, they can take less for granted, and have a diminished ability to rely on brute force to make up for their shortcomings. Compared to Aginor, Balthamel, Ishamael, Rahvin, Lanfear and Bel'al, they have done extremely well for themselves in terms of success and survivability.

We know, for example, that Mesaana is one of the few Forsaken whose work kept having an impact long after she was Sealed, and made the Breaking worse because Mesaana's Children were still causing chaos, whereas as soon as some Forsaken vanished, they just were no longer mentioned. Even Ishamael shows his relative lack of effectiveness - if Graendal, Demandred or Lanfear had been able to leave the Bore like Ishamael had, I tend to think they would have been more effective at setting things up. At the end of the day, he really didn't accomplish all that much despite having this advantage over the others.