I'll admit I haven't watched any HDR content in mpv yet, but it supports all the standard tone-mapping algorithms (hable, reinhard, etc.) and apparently the default algorithm (mobius) was chosen for its colour accuracy.
I'm not saying madVR doesn't have its strengths. Error-diffusion dithering is strictly more accurate than what mpv does, and for people who like super-resolution upscalers, madVR tends to have a larger selection and faster implementations. Still, it would be disingenuous to say that madVR has a larger feature-set than mpv. They have different feature-sets. mpv's convolution-based upscalers are much more tunable and its colour management is more advanced, since it can use an ICC profile to auto-generate a LUT for any input gamut, rather than relying on an external CMS for this.