I think it's rather harsh to portray Luna as "using" Mai, rather than her wanting to have a normal romantic relationship where she can fall in love too. It's made pretty clear that Luna was hoping for their relationship to make her feel something in return, but she didn't, because she's (probably) aro/ace.
It's not that she used her, it's that she came to realize that their relationship couldn't be the way either of them imagined it, because Luna couldn't fall in love.
I find it really disingenuous to act like Luna was doing something wrong or being malicious when she wanted to fall in love and have a proper relationship with someone. Relationships don't always work out the way you want them to, and in this case especially, neither of them are at fault. Just because Luna couldn't feel the same way doesn't mean she used Mai.
To begin with, doesn't everybody "use" the person they try to date to figure out if they want to date them? That's how dating works. Sure, some relationships are more casual than that, but if you're trying something at least semi-serious, you start by dating to see if you're compatible, right? Nobody knows until they try. Luna tried, and it didn't work out.
Even people confident in their allosexual alloromantic identity are going in blind, because relationships are complicated and messy and it's not like you get a manual. People who are on the aro-ace spectrum don't get born knowing
where on the spectrum they fall; a certain degree of life experience is needed to understand yourself. From the looks of their clothes in the flashback they started dating as teenagers too (not that undergrads are that much more worldly).
Luna did err in not communicating this, and probably being too insensitive in asking Mai for help in clothes shopping for her rebound date. That's her failure. But failure isn't the same thing as malice.