Is this an example of pathetic fallacy in "Jane Eyre": "Nature must be gladsome when I was so happy.”?
Yes, it is an example of pathetic fallacy. See explanation.
Pathetic fallacy is a literary device which describes a scene comparing the enviroment to character's feelings.
The given quote describes that the nature is gladsome reflecting the character's happines.
In otherr words we can say that the nature is "as happy as the character"