I'd imagine you can definitely make decent maps and still suck at modding. Since you need to be able to explain briefly and convincingly why you think things needs to be changed, while still not being so stubborn that you force your own mapping style onto others.
with mapping you can easily just get into refining a single style and make good maps, while with modding you can't just try and make everyone fit into your box as there isn't a best way to map.
same thing happens with other skills. not everyone good at math makes a good math teacher, not every athlete can be a coach, etc.
a better metaphor would be asking an artist to critique your art. They are really good at their art, but maybe you do something different that they don't do, or they aren't very well spoken and cant explain their thought process. There's a lot of things that they would need to know how to do outside of art alone to give others constructive criticisms.