Creeping Jenny loves a moist to wet soil so I'm thinking bulbs planted below would likely rot from the moisture.

The old foliage from the daffodils would kill any parts of a low-growing plant that they flop onto. If by creeping jenny you mean the weed, it wouldn't be an issue, but if you mean Lysimachia nummularia 'Aurea' it would leave a dead spot. If you don't let the bulb foliage ripen, the bulbs won't return well. I find my bulbs do better when they grow under taller perennials that will hide the foliage and won't be killed by flopping bulb foliage.

NHBabs, what do you find are the best perennials to grow with daffodils?

tete_a_tete, I have daffs growing with Helenium, daylilies, Baptisia, perennial geraniums, and roses among others. All tall perennials with the bulbs planted between the perennials so as to help hide the bulb foliage.