Yes and no. You are right that Ubuntu Mate should keep its simplicity and lightness, and work perfectly in underpowered or old computers.
But on the other hand, if we consider the fact that many people use online services of big companies like Google, Microsoft, Facebook, Twitter, etc, without online accounts integration the user is forced to use the web versions. That means you have to use web browsers, which are big consumers of resources, both CPU and RAM. And there we enter into conflict with the lightness and little use of resources needed by older or less powerful computers. Using Chrome or Firefox to check the calendar, contacts, etc, it’s an unnecessary waste of CPU and RAM.
I think it’s possible to have both, lightness and functionality, the user is who benefits, and sincerely, the user is who ultimately matters. A distro without users is doomed to failure.
In this sense, intending to save RAM usage would also be a good idea to change Thunderbird by Geary, an email client modern and more efficient. In my case, Thunderbird plus five accounts and calendars synchronized uses in my laptop between 250 and 300mb of RAM.
It’s just my opinion, based on my daily (work & personal) use of Ubuntu Mate.
Best regards!!
PD: I apologize for my English, is not my language.