Is there a way to undo/reverse a pipeline run?

No, there is no "undo" button like Excel has or a CTRL-Z in Typing into Word.  I think that your only option is to get into a discussion with Support about providing you back a copy of your app as of a certain midnight or CSV files from previous nightly saves. They retain a nightly backup of your app every night for 14 nights and then every Sunday for six months. 

The you you would need to manipulate that data to update the records back to the way they were. 

For example if the key field of a table was Record ID you could import the Record ID and the field(s) which got corrupted back into the app and the old data will merge in on the Record ID as the key field.