Guitar Hero II Strumming Question

[ChipsAhoya]

I recently completed Hard in career mode but only got more than 3 stars on 5-10 songs or so. Since I have no musical training whatsoever, I have simply been strumming down using only my thumb every time I need to play a note. Is learning to strum up and down with the thumb and forefinger absolutely necessary or should I just be working on improving my hammer ons and pull offs?

Thanks,
ChipsAhoya