She is tall but smart versus she is tall and smart.
1. She is tall but smart.
2. She is tall and smart.

What's the meaning of the sentence one, does it mean "she is tall and stupid."?

Question

She is tall but smart versus she is tall and smart.
1. She is tall but smart.
2. She is tall and smart.

What's the meaning of the sentence one, does it mean "she is tall and stupid."?

Answer

This sentence is fine if you perceive the speaker is thinking in a particular way:  
"She is tall but smart" = even though she is tall (and in this instance perceived as a negative (quality) for the speaker), it is offset by the fact that she is smart.

Answer

The first sentence does not make much sense. It means that she is tall, and in spite of that she is still smart. However we know that being tall does not stop you being smart, so the sentence is not right.