I don't think the issue with your feedback is that it is unkind, but that it is not specific.

"Cheap" is not a type of design.

"Probably a quickly modified free template" does not say what you found wrong with it.

"Unreadable" is just, on the face of it, false since I could read it.

If you had been specific with your feedback -- even if it were negative -- I think it would have been better received.