Should plugin reviewers be paid? If so, why not pay the theme reviewers?

As a Theme reviewer: I say absolutely not. The WordPress project likely doesn't have the funds, and WPF might have a difficult time justifying such an expense as a 501(c)3 organization. So: who ends up paying? The only viable source is Automattic, which would be an inherently bad idea, IMHO.

Maybe theme and plugin submitters should pay a fee for reviews?

Again: absolutely not. It is imperative that reviews remain fair and objective. Introduce financial incentives/contributions from Theme/Plugin developers, and that fair/objective standard becomes infinitely more difficult to maintain. Heck, the WPTRT has already had to defend itself against accusations of taking money for approvals, and/or for the Featured Themes listing; imagine if actual money was involved.