Well, afaiu Mer/Nemo (similar to MeeGo CE) aims at providing a full Open Source smartphone/cellphone OS. This includes not only the "standard" Linux kernel + user land stuff but also Open Source implementations of vital cellphone functions such as making calls, 3G, or SMS functionality.
While Maemo 5 (Fremantle) as well as Harmattan are to some degree Open Source, the vital cellphone functions such as calls, 3G, SMS etc. are closed source bits. iirc with Maemo5 it's even worse as there are quite a number of closed source parts that are very important to the entire system (bme, e.g., was some keyword iirc).

So in this sense, yes, Mer/Nemo are a pretty big improvement over what we have with Fremantle or Harmattan nowadays Open Source wise. As far as I could follow this whole topic, having Open Source implementations of this cellphone specific stuff is really challenging. And while it may seem quite "trivial" to have such functionality like making calls or sending SMS it may actually be pretty hard work to have these bits implemented as Open Source.