Do you need to install DXB Smart App for transiting through DXB airport? I installed it anyway on iOS and tried to register but it hangs at "Please wait" and then just does nothing.
The app seems to just be a way to get information on the covid situation in Dubai, and contact the local health authorities if necessary.
Are you required to use the app? It's hard to prove a negative, but I see no evidence. Beyond the fact that it would prevent anybody transiting who, for example, doesn't own a smartphone:
Absolutely nothing on the airport's info page for connecting passengers suggests the app is required for passengers transiting through Dubai airport. The app has been available since at least April 2020, so it seems safe to assume that a major airport like Dubai would have updated the information by now if use of this app were in any way required.