There's quite a bit of processing that needs to be done to import a web page, images and styling into Joplin and it would be tricky to get it working in mobile. Also these days you have to load the page in a headless browser to get the proper rendering and that also would be hard on mobile.
The current sharing system attached a url to the note. A plugin on desktop could be written that does the clipping based on that url. It's not as friendly as clipping on device, but it would get a better clip.
There's quite a bit of processing that needs to be done to import a web page, images and styling into Joplin and it would be tricky to get it working in mobile.
I had opened a issue on Github where you mentioned that clipping simplified pages uses "Reader View from Firefox" libraries. Since Reader View is also available on Firefox mobile, it should be doable to clip simplified pages from Android client-side right?
The difference as I understand is that in browser the lib has access to the rendered page which for instance can be created dynamically by scripts. Doing the same in mobile client requires implementing a significant part of a Web browser essentially. It's probably doable but creates tons of issues e.g. with security if we allow executing code from arbitrary webpages.
Here's a fun experiment - turn off javascript in your browser and browse your favorite website. This would be roughly what a naive implementation of mobile Web clipper would get