A .bag file is just an store of serialized ROS messages of any kind. It has no affinity for any particular type of message (such as PointCloud) and so you can't open a .bag file in any application unless that application is specifically written to read a .BAG file, and for that, it would need to incorporate the ROS C libraries that read and write to bag files (http://wiki.ros.org/rosbag/Code%20API).
So, the answer would be no. The main app that opens bag files is rqt_bag, a plugin for the rqt ROS GUI framework. If you want to visualize a pointcloud, you can do it with RVIZ fairly easily, but I appreciate you may want to edit it or otherwise analyze it. One thing you can do, is just do a "rostopic echo and pipe the output to a file. The resulting data is very easily modified in a text editor to become a .ply file. You can look up the .ply specification, but it is just a little metadata and all the x,y,z points that constitute your cloud. Be very quick to hand-modify the rostopic echo data into a .ply file and then you can import into meshlab or whatever you like.
Originally posted by codenotes with karma: 261 on 2016-02-06
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Original comments
Comment by CLDer on 2021-05-14:
Any hint on how to "pipe the output to a file" when doing "rostopic echo "?