The design patent which the question references protects the distinctively recognizable look of rear-view mirrors protruding out from the end of bicycle handlebars. The mount in 180° straight out from the end of the handlebar tube, which creates a quite distinctive look that is quite unusual.
The question seems to be describing a way of defeating this distinctive look by not mounting them at the end of handlebars, but rather on the handlebars further toward the steering stem of the motorcycle. The question seems to predicate this on a change of angle from 180° to instead 90°.
In my opinion, the design patent had nothing to do with the shape of the mirror itself as a reflective surface, but with the look of the distinctive placement of the mounting via the end of the bicycle handlebar with the mirror at a sharp angle (e.g., approximately 45° for anyone who has utilized this bicycle mirror in practice, as I have on my own bicycle and my wife's). The question clearly defeats all the distinctiveness that the design patent conveyed and that that style of bicycle mirror has in field deployment. The question in design-patent infringement (much as with trademark infringement) is: will the consumer confuse the supposed infringer for the design-patented original. I would say that most juries would say that mounting via a 90° bracket further toward the steering stem in no way resembles a 180° bracket protruding out the end of the handlebar. Most people would say “Whoa! what is that thing protruding out from the end of the handlebar?” in the design patent, but say “Yeah, garden-variety customary mount of a mirror onto the interior part of the handlebar.” for what is described in the question. Hands down no infringement of the design patent in the question as described.