Think of the term 絕世高手  (Master out of this world/ extraordinary master); 绝代美女 would mean (Beauty out of this generation). Which means you may only see beauty like this once in a generation. It is an exaggerated adjectival phrase, a more realistic version of this phrase would be 'top class' or 'extraordinary' 

*Exaggerated adjectival phrase is very common in Chinese, for example, 惊天 (shock heaven) is an exaggerated phrase for "shocking"; 万能 (ten thousand usage) is an exaggerated phrase for "multi-functions"*

"太多绝代美女" means "there are so many extraordinary beauties "

As for your question, depend on the context, "什么叫" can mean "what calls" (with "what" as the subject) or " what is called" (with what as the object)

In this context : "什么叫绝代美女?",  she is asking "what is called extraordinary beauties?"

The difference:

- "什么叫绝代美女?" (what is called extraordinary beauties?) is asking what qualified someone as an extraordinary beauty (asking for your opinion, implies some doubt)

- "什么是绝代美女?" (what is extraordinary beauties?) is asking what is the  definition of extraordinary beauties (asking for a definite definition)

Since you claimed Ukraine have many 绝代美女, she was asking you to make clarification on this statement.

She knew what the term '绝代美女' means-- "an extraordinary beautiful woman; a rare beauty"