Is the GLES2 backend in the works also cover the web target to be able to target less powerful devices?
Godot 3.1 will reimplement an OpenGL ES 2.0 renderer, thus, WebGL 1.0 will hopefully be reimplemented into the HTML5 backend when 3.1 is released. Note that as of Godot 3.0, only WebAssembly is supported – I doubt asm.js will come back (it only matters for Internet Explorer 11 support).
Is the web target also suitable for mobile?
I don't remember ever playing an HTML5 game that was smooth and responsive enough to be enjoyable on mobile devices, so I doubt it.