Even if death and fear have their place in "The Bells," there's also a lot to smile about. After all, for the first 35 lines, things are going really well – which is really weird for a Poe poem. The whole world seems to be singing with joy and harmony. Poe is almost never cheerful, so we think it's important to point it out. Also, at the end, the ghouls feel some happiness, too, although in this case it's a much darker joy that they get from causing despair.
The poem makes it clear that happiness is not just a single feeling. In particular, "The Bells" makes a distinction between excitement and fun, in the sleigh-bell section, and real harmony and joy, in the wedding-bell section.
By showing us the dark happiness of the ghouls, the poem suggests that feelings of joy are not enough to make happiness. It must be combined with peace and harmony.