Most of us use these kinds of sentences “It’s too hot”, “ It’s too rough”, “It’s too high”. We don’t realize that the proper grammatical construction is “too…..to”. It should be “It’s too hot to go shopping”, “ It’s too rough to be worn on bare skin”, “It’s too high for me to reach it”.
But in a spoken conversation, we tend to leave out the second part. It’s taken to be understood by both the speaker and the listener. The sentence given in the example should have been “She is too cute to be true”.. she is so cute that she cannot be true. No one can be so cute. Or it should have simply said, “she is very cute”.
Hope this helps.