Why does the Tu-134UBL have a sharp/pointed nose cone?
According to Wikipedia, the UBL variant of the TU-134 was designed/used to train TU-160 crew, and is using its nose-cone.
As to why, I think we can only assume. I think that a fair assumption would be that it contains the radar from the TU-160 (or a stripped-down version of it), and/or so the crew will have a similar visibility as they would on a TU-160.
The overall performance differences between the TU-134 and 160 are too far apart (the -134 being a subsonic airliner and the -160 being a supersonic bomber) for the shape of the nose cone alone to make any sort of a significant difference.