theme is just one rating category :) you'll generally be rated lower if you don't include the theme at all, but if it's one of your first few jams you shouldn't compare yourself to other games anyway!
Just pick your best idea, even if it fits loosely the theme, rather than picking a not so good idea that fits it better.
Worst case scenario, you are rated poorly on the theme category, therefore the general rank suffer from it. But it doesn't really matter as there are nothing to win anyway. And you end up with a better game that is most likely to do well after the jam.
Best case scenario, your game is good enough and the other rating categories compensate for the relativelly low theme rating. And as people tend to overate good games, the theme rating may not be that low anyway.
For example my entry on the last ludum dare got first place althoughit was not a good fit to the theme.
