If you aren't sealing the top of the condom opening with tape then there's no point to even using the condom as the sweat will just collect in the condom and then the condom helps the moisture stay on the mic pack.
In my experience, a condom with a cotton ball on the top are useful only for fairly dry actors. It helps to keep residual skin moisture off of a pack when an actor isn't really sweating, but that's about it.
If the cotton ball (and whatever powder is lining your condoms) isn't able to absorb all the moisture, then trying to seal the condom (or the top of the pack inside the condom) can be helpful, but for real sweaters (think choreo-heavy shows in un-air-conditioned venues) I'll often resort to sealing the packs in a layer or two of Glad Press-n-Seal. I've also heard good things about sealable neoprene pouches.
In the end, it's a matter of finding the right solution for the problem. I'll always start with a condom and cotton ball on everyone, but any sign of moisture on the pack means escalating to some higher, more sealed, solution, until the pack comes back to me as dry as it left.
-Russ