Plain reskinning is something I'd call counterfeit (I know some systems that ripped off NES games while changing the graphics, like the N-joypad, 0 original games), under no circumstances it can be considered good (it is a ripoff, and can potentially harm the original devs).
Redesigning an already existing game (assuming the license let's you do it) entirely, like levels and such is an approach that can be considered fair, however improvements are needed, and maybe enough changes to make it more a standalone than simply a mod.