It's not selfish at all..If I recall, it's no different than when Mordin or Jacob or whoever it was that said people were taking care of business because it may be their last chance to close any loose ends. As for joining the crew, yeah and Zaeed could've easily joined the crew as could Grunt and/or any of the other squadmates yet that didn't happen. It's funny you mention Joker and Garrus..Joker who was focusing on wanting to hookup with EDI and Garrus who was concerned about his family doesn't really support the accusation of Miranda being selfish. Joker said something along the lines of it's the end of the world yet people are still going to the movies and stuff because they want to believe that things will be ok
Mordin or Jacob or whoever? I never made any excuses for their actions, but if you insist.... Mordin did rejoin the crew, and intended to stay until the reapers were defeated, but had to leave because he either died or had to fake his death for the krogan/turian alliance. Jacob is an ass. Zaeed is a selfish mercenary who you should never expect any altruism from.
And... wtf did you say about Joker and Garrus? Joker cannot have any personal interests while performing his duties? It has to be all-business 24/7, even between missions ...just because? And Garrus being concerned about his sister is not the same as abandoning fighting the reapers and the Normandy to look for her. Big difference there. It's sad I have to point out such an obvious contrast.
Lastly, Joker was referring to civilians (who aren't part of this argument) in that banter, which Miranda clearly isn't. Again, I've made no excuses for those people's behavior on the Citadel either.