Ball Weevil itself is just a talking bug. Ignoring the fact of how funny (and somewhat cute) he can be, he's still a talking bug. He doesn't even has arms or anything. He could be strong like some insects are, being able to lift hundreds of time his own weight, but I don't imagine him doing so.
All of Ball Weevil's strenght comes from his plasma balls. He's actually able to handle major enemies, but he needs time to create potent enough balls and he's not as effective as other aliens are.
Conclusion: he's a medium alien, not too weak or too strong.
personally, i like to see all of ben's alien heroes are ultimately be equal becuase they al lof their uses in specific situations as wells as that some of the most dangerous aliens like "alien X' and "ghostfreak" have strong weaknesses and shortcomings that bring their usefuless down a notch.