Mannerisms, of all kinds, are essentially, Learned, Practiced and Acquired human behaviour.
And Acquired and practiced behaviour, essentially, is a way of acting, speaking and responding so as to portray a Persona…

Which, while is useful in making human interactions smoother and more civil, by a collective agreement to adhere to a minimum standard set of behaviour, so as to avoid hurting, offending or annoying others in a society…

But beyond that, it also unfortunately implies that it is actually an agreement to adopt a façade that is intentionally practiced to appear as one is not.

For better or for worse, it is but a deliberate attempt to make others think of oneself as a person one is not, thus, to be brutally honest, can be contruid as an attempt to decieve.
On a deeper and more existential level, Spontaneous Response, is a much higher value, and a lot more sincere way of relating with another, specially in the context of relationships and Connections which are more than merely social or incidental.
To be impressed with mannerisms, is to be impressed by how well one can Act,
To be impressed with Spontaneity, is to be impressed by what one IS.