The first time I went to Muji in 2002 they barely sold clothes, when i went in 2015 they had a much larger clothing section, the clothes are were pretty grey and basic just what one would expect from muji. As others have said its basically a japanese dollar store but its not cheap.
Uniqlo is good, nothing exciting, pretty good for basics. Quality is ok, the socks i have didn't last that long however.
uniqlo - clothing store
muji - fancy dollar store, furniture, household items, small electronics, great storage, great stationary, clothes, bags, etc.