I don’t think the blame is on you In the Dear ImGui site they have a zip file with pre-compiled binaries and both the OpenGL & Vulcan ones show the same GPU usage behavior. So the issue is more fundamental in its origins.
Nicolas Guillemot addresses the power inefficiency in his C++ lightning talk on Dear imgui. It is an immediate mode renderer that recreates the entire gui every time through the event loop. It is very responsive, but power hungry.
Had seen references to it but didn’t realize the effects of that continuous recreation were so drastic.