Welcome to the Actors Forum!
I think you share a worry with many other individuals out there with their hearts set on acting. But, SWK78 is absolutely right. "Beauty is in the eye of the beholder" and no one has the right to take it away. The bottom line is: talent agencies are looking for ... talent!
An average looking person can be given super model looks overnight, but learning how to act involves much more. To attain talent, one needs to do more than take in information and put it to work. Setting goals, receiving immediate feedback and focusing as much time on techniques as on the end result all contribute to the development of one's talent.