Do people really care if the game matches the theme or can I just make whatever I want?

I don't think I can fit the theme to any of my game ideas.

Since the Theme is one of the rating fields, your overall rating may suffer from it. Personally, I wouldn't knock points off of any other rating though. 

You don't have to match the theme it is just one of the rating fields so like QueueDevelop said your rating will loose points for that but your game will still be valid and gain points in other field.

It’s write on the main page: “The games will be rated in 6 categories: Game Design, Fun, Innovation, Theme, Graphics and Audio. This is a wide variety of game dev aspects, so don’t worry if you can’t hit all of them perfectly! Try to be creative and stick to the theme though!”

I'm pretty sure that sticking to the theme is important, mostly because if you ignore the theme, it would just be a competition on who makes the best game.
The theme gives limitations on what the game can be about.
So I think that if you want to try and score high you could simply make a game without time, but I'm pretty sure that you will not win nor get high in the rankings if you don't follow the theme someway.