I agree. When registration and login forms are displayed together, the login forms is pushed down.
However, I still have not found the solution to this, neither am I willing to change or try to fix this issue by changing the theme. But I have found a temporary solution.
I have disabled the login form and the only way to login is via website user account only. what I found is that, when a person signs up on affiliate registration form, he is automatically registered to the website, so the next time he visits the website, he can sign in the website user account and then visit the affiliate dashboard page.
It just adds an unnecessary step to visit dashboard, but it works for now until YITH developers look into this problem and actually solve the issue.