Yes, the user can edit the details which are pre-filled.
If you don't want the user to be able to edit the data, you can make them read-only.
To do so, append parameters data_readonly=<<variable name>>
For Example:
instamojo.com/SampleUser/SampleLink?data_name=Jerry+Seinfeld&data_readonly=data_name&data_amount=1000&data_readonly=data_amount
Note: You can keep any of the fields as read-only.