reflective tape works well on flat surfaces, but mudguards are curved in both directions, so unless the tape is stretchy it wouldn't stick neatly.
Thin tape/sheet is often just about stretchy enough to make it work, albeit with the odd small crease at the edges in some cases. Mine doesn't look 100% neat close up, but stays stuck down perfectly well. It helps not to cover the whole width.
As for being visible, reflective tape can be *more* visible than a "proper" reflector even when it's less reflective per square inch, simply by covering a many more square inches. The effectiveness of commonly available products, IMO, ranges from adequate to amazing.
I even made mudflaps entirely out of stiff reflective tape - not one of my most durable creations! Of course sticking the tape on sexisting flaps is the way to do it.
Is this some kind of kinky bondage you're into?
Sorry*. Of course I would never mean to recommend sexting one's flaps.
* Not really. I noticed the typo before posting but posted anyway.