When looking at accounts on my profile, we have a generic logo. Lots of other stacks have cool designs.
Is anyone accepting design concepts for an IoT logo?
You're absolutely right that many sites have nicer logos - and actually full themes - than we do. The reason is because we are still in beta. As it stands currently, beta sites don't have a full site theme, and all follow the theme mentioned in this post on meta.
There is, however, a tentative change coming, in which there is supposed to be a fairly major overhaul of the themes. There's also a little-noticed comment by Jon Ericson which suggests that beta sites may be able to work out their own themes once the changes have been rolled out.
So by and large, the answer is, we can't change this; we have to wait on the Stack Exchange team. Hopefully, they'll work something out. Probably in about 6-8 weeks.