lxde inverted mouse scroll

You should actually use

$ xmodmap ~/.Xmodmap

to load the configuration. If not at your home directory when executing

$ xmodmap .Xmodmap

you probably get

xmodmap:  unable to open file '.Xmodmap' for reading
xmodmap:  1 error encountered, aborting.

If that still doesn't solve the issue, Natural Scrolling in Lubuntu 17.04 has several alternatives, which seem to have helped some. A summary:

1. Modify ~/.Xmodmap and reload it, as described above.
2. Edit /usr/share/X11/xorg.conf.d/40-libinput.conf as shown here, which includes both the pointer and touchpad.
3. If this does not work (100% of the times), try editing /usr/share/X11/xorg.conf.d/70-synaptics-conf as shown here.
4. Try changing gsettings with gsettings set org.gnome.desktop.peripherals.touchpad natural-scroll true or gsettings set org.gnome.desktop.peripherals.touchpad natural-scroll false. You can also do this via gui, with dconf-editor (may require installing it).