An argumentative thesis statement makes a clear position on whatever topic the writer so chooses. The best thesis statements tend to be clear and to the point in presenting the main idea of the essay to come.
From your question, it sounds like you have not exactly decided what your argument is going to be. In this case, it might be best to ask what element of Frankenstein are you applying to psychoanalytic theory? Once you have selected what Freudian idea you're applying to Frankenstein, then it's time to make an argument about Shelley's text using that idea. For example, a paper arguing that the creature is a projection of Frankenstein's id should not have a thesis statement that just explains what the id is in Freudian psychology, because that is not an argumentative claim. All that would be is a definition that cannot be debated. However, the notion that the creature represents Frankenstein's id is a claim which can be debated and so makes for an appropriate thesis statement.
Specificity is another element needed for a successful thesis statement. It would not be enough to just say, "The creature represents Frankenstein's id." Going into a brief description of why helps make the thesis stronger (though do try not to be too detailed, since the nitty-gritty of your arguments will be in the body of the essay). So a thesis like "Due to his emotional and violent behavior, it can be argued that the creature is the id to Frankenstein's ego" would be more impactful.
All in all, try to just narrow down what it is you are trying to argue. What point are you hoping to convince your reader of by the time they finish reading the essay? Once that is clear, coming up with a thesis statement will be much easier.