Your right that the merchandise lacks inspiration.
Why doesn't Tomato do more work with UW? Its probably because the collective has changed - the core people have moved on or are busy doing other things... they have nothing to gain by doing free work for their UW friends. These days Tomato is more like a distant relative to UW than a partner.
In general, Underworld seem to have been cost saving as much as they can since they were with any major record label. Everything seems to be a lot more low budget - The live on stage setup, the website, the merchandise and even some of the physical releases (for me there is something slightly "home made" about them compared with other things in the record shops). And not to mention some of Karl's
home made shirts 
Its not a criticism, just an observation - this is the only way bands can survive without the financial backing of a major label by keeping the costs down, and not selling merchandise at a loss. It's about running a business as well as making great music.