Navi Mumbai is pretty clean, though its stations aren't what they used to be once.
Sanpada is good, Vashi, juinagar are above average, khargar needs improvement and Nerul needs a lot of improvement :P
but the city is still very clean, you can feel the difference once you cross over one of the 2 bridges to get into Navi Mumbai, both at Mankhurd and Mulund.