Thinking on it, it makes sense she's insistent. For one, his species may not need bathing much, but there could be alien bacteria on him that could do weird things to the others. Plus to protect him - Maybe some earth bacteria can breech his skin's defenses. So he'll actually have to bathe regularly until "Mom" can figure out the microbial and viral interactions between the species
It took longer than anyone expected, but Earthlings finally set out into space. When they discover superlight technology and set forth from their unintentionally hermetic world, one particular galactic observer organizes a rendezvous with the Earthlings, and is greeted by an unusual surprise.