Is it ethical to request additional money for non-research related work in a research lab as a grad student?
From one perspective, adding these niceties to the software increases the likelihood that the software will be used and thereby increases the impact of your research. So although these niceties are not research per se, they help achieve the goals of doing research (at least from a funding agency's perspective), namely making an impact. Such tasks are often a necessary part of the research process (and someone has to do them).

Whether or not it is ethical to ask for additional money to hire people to do this very much depends on the rules of the funding agency and how well your advisor is at selling and justifying the idea to the agency.