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Because this is a presentation layer aspect, I'd suggest that we look at this mainly from a presentation layer scenario. As kris_fr said, tellling people which of the popular jscript frameworks to use (jQuery, mootools, scriptalicious, etc.) is a bit authoritarian.
However, in terms of solving the problem of making XOOPS a shiny OOB experience like other CMSes that get more limelight because of that one fact (shiny on the outside ugly on the inside), solidifying on such a toolset can be a positive move and developers can provide templates to match that capability. But make sure it remains in the presentation layer.
Further, to be good netizen developers, my recommendation is to load both jQuery min and jQuery UI through
the Google API loads. The positive side is that since many sites use these already, users have it cached in the browser making for a faster experience overall. Also, since we'd be using just the min loads, then developers can decide which other plugins they want to use on their site.
As a caveat, some sites I've built are for people of disability (can't use jscript, flash, etc.) so must be able to disable it and/or work without it.
I've been using jQuery in my last several installations, and it works great!