Why does the viscosity of honey decrease when mixed with other viscous fluids?
Honey is a homogeneous substance, a solution of around 98% sugar and water.

Mustard is a heterogeneous mixture, containing about 1:1:1 oils, proteins and water, all in very small particles. The proteins act as a stabiliser to the small oil droplets. They cant slide past each other easily, and act more like a dense network of small solid particles. Like a heap of wet sand.

Now if the aqueous phase gets much larger due to the honey, the oil+protein droplets loose direct contact, and you no longer have the solid-like behaviour of wet sand.

Your aqueous honey-phase however now contains perhaps 25 or 30 percent of water instead of 15 or so in pure honey, and that lowers its viscosity a lot.

You have a major phase with reduced viscosity, and a disperse phase which is no longer continous (not "percolated"), and so the whole thing flows rather easily.

(Btw. the viscosity of mustard is not that high, however it usually shows a yield stress, that is it does not flow at all e.g. under its own weight. You open a mustard glass and the inside looks just the same as it did when you last closed it. Honey however is just a boring Newtonian fluid, no shear thinning, no thixotropy, nothing.)