How come the expansion of the universe only applies to space and not mass? And if time was also created at the Big Bang, how come it doesn't apply to time? Or could it be that it does, but we can't measure it since there is no other instance of time to compare it to?

It does apply to mass. However, much mass is bound together by forces which can resist the expansion over small scales. So, whereas large galaxy clusters are moving apart, galaxies remain intact, as do stars, planets and atoms.

Some individual particles can be effected by the expansion of space. Light from distant galaxies is red-shifted. This is not because the distant galaxies are moving away from us rapidly through space (that doesn't really make sense anyway), it's because the space between us and them is expanding, and light that has travelled from them to us has been stretched too. As light is stretched, its wavelength increases.

As for why space and not time. I have absolutely no idea. I am going to look more into this.

And are these questions stupid?

Absolutely not.