To factor out a function (which is useful when the mapping commands become longer), the only thing you need to consider is that the special keys like <Esc> that work in the right-hand side of the mapping don't work inside the function; you need to use :execute with double-quotes, and escape them:
function Linecomment() execute "normal ^i/*\<ESC>$a*/\<ESC>" endfunction command Linecomment call Linecomment() To comment from the cursor position, you just need to drop the ^ motion at the start.
However, for a truly powerful and robust commenting solution, I would recommend to use one of the popular plugins: