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?"
"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"