I think what she's wearing is a crap choice for travelling by plane. You're crammed in, & no one wants a stranger's bare shoulders rubbing against them for hours.
I'd feel the same about a sweaty bloke in a vest top. Just put a proper tshirt on ffs.
Perfectly ok beach/casual bar wear. Just not ideal for being in close quarters with strangers for hours in a narrow row of seats.
I think the airline over reacted, though. It's not offensive, just not ideal.