It's just a bad idea, a distraction at best.
I've been playing with networking devices for quite a while now, both highend business gear with its own windows configuration tool and, contemporary highend consumer devices which really emphasize the android/ iOS app idea and quite a lot of inbetween… Aside from the highend business gear, the provided tools have never really been full featured, despite having a much narrower focus than OpenWrt, so they have always been a dumbed down selection of basic configuration options.
Now, what would you actually win with such a smartphone app?
- less ressource usage on the router?
- no, not really
- you can't really do away with the webinterface completely, acting as the lowest common denominator
- uhttpd and luci isn't that much of a ressource hog either, if that is a problem in your eyes, you have bigger fish to catch.
- a better user experience?
- perhaps for new users, my experience with already existing commercial apps of this kind however doesn't really suggest a major improvement, at least nothing beyond what an optimized CSS skin would provide
- nice guided assistants?
- there is no technical reasons why those wouldn't be possible to write for luci either. Writing good guided assistants is just a lot of work, but not more (or less) than doing the same for a smartphone app.
- just to have a nice app icon in your launcher?
- this I could actually understand, to some very limited extent. It would be quite nice to have a small launcher on mobile devices, which remembers connected devices and caches login data (apart from generic browser password caching), but personally I'd leave it at just that - everything else handled by the ordinary responsive webinterface (on mobile devices, the OpenWrt2020 theme is actually an improvement.
The way I understand the question under discussion however would be a nightmare for me personally. Leaving aside the questions of how to (ever-) make such an application feature equivalent to the webinterface or how to allow external contributors to allow development of new functionality and features, having to write an 'app' would mean doing everything at least three times, once for the webinterface, once for android (and there is android/ ARMv6. android ARMv7, android ARMv8, android/ x86_64, android/ mips), once for iOS, immediately trippling the development efforts (as they won't share a lot of code).
But the suddenly much higher development efforts aren't about my nightmares - my nightmares are about interoperability. As mentioned before, I've been playing with a wide spectrum of networking devices for over 3 decades now, I've seen standards come and go. My memories of ISDN pbx systems requiring RS-232C connections and DOS configuration tools aren't too fond, neither those ISDN pbx systems that were USB based, but whose tools only worked on Windows XP-SP2. Or the switches that hard-depend on JAVA JRE1.2 or MS ie 5.5. Some of these still cross my path today, way past their experation date - and it's not funny to keep virtual machines with ancient software at hand, nor to keep using notebooks with native (USB emulation isn't good enough) RS-232C interfaces at hand. I don't even want to be remembered about Windows CE or Windows mobile/ Windows RT (ARM). Especially 'invisible' infrastructure devices have a tendency to stick around for a long time, it is an advantage if those can be configured by a 10-year-old notebook with an equally ancient browser just as well as with the zPhone XXX from the year 2033, ~if man is still alive, if woman can survive, they may thrive~.
Yes, a webinterface might not be as hip or sexy as native smartphone app, but do you really need a self-concious, cloud backed smartphone app, which live tweets how bored it is?
 Neither of these can really do without a webfrontend completely, even if the primary focus is on their vendor based tools.
 where the alternative webinterface was really something to 'get used to', albeit technically full featured; contemplated with proprietary ssh/ telnet shells.
 OpenWrt comes with a lot of luci apps, plugins which hook into the common core without having to be officially blessed, adding features that would never be part of any smartphone app.
 well, at least if you concentrate on a single smartphone eco-system.
 or want?!
 quick, need to add a.i. acceleration and blockchains somehow
…and now get off my lawn, young whipper snapper. Back in my day, we walked uphill, both ways, in the snow to school - and we were happy with it