is the Sobolev space $H_0^1(\Omega)$, where $\Omega$ is a bounded 1D interval, a Hilbert space?

Yes, it is Hilbert. The inner product can be taken to be $$ \langle f,g\rangle = \int_\Omega f'g', $$ or perhaps more naturally $$ \langle f,g\rangle = \int_\Omega (fg+f'g') . $$ These two inner products induce equivalent norms for $H^1_0$, as per Friedrichs' inequality.

Note however that the first one induces only a semi-norm for $H^1$.