If a teacher gets frank and friendly with students, is it going to mar his performance?

YES, unless and until it is  really required , the teacher should not get friendly.

it can mar his performance if it leads to lack of respect. So it depends on the way the teacher is dealing with the students. To be friendly is good but he has to pay attention to keep distance and respect.

From my opinion and point of view teachers should not be too friendly towards their students. On one side the teacher keep a boundary between him and his students. The teacher should make himself be respected on one hand so even if he is to joke with his students (at times it is needed as to awake the class) they should know where their limits are.

Every teacher has its own way of teaching, some time he is hard on them and some time frank with them according to the class atmosphere.

so it does not mar his performance.

yeas ofcourse