Well, it's pretty simple if you've seen the anime for a bit.
Ash and the gang (depending on whatever series) have always escaped from Team Rocket's plans, and they have never successfully stolen a Pokemon from them. And, according to Google Dictionary, the definition of twerp is:
A silly or annoying person
And, quite frankly, the gang are rather annoying in Team Rocket's point of view. So, I guess that's my answer. xD
Hope I helped. :)