I find something cheesy if, instead of eliciting the desired response, like 'awww, how sweeet' or 'gosh, that's romantic', it makes me shudder. Movies do this a lot, especially ones aimed at kids.
I think the magical armour would be cheesy if there were simply no basis for it in the story. If the person summoning it hadn't had to learn or sacrifice in some way to be able to do it, if there were no cost, and if it appeared all shiny and complete in the middle of a dirty, nasty fight. But I'm not sure. It might equally be because she's surrounded by some kind of halo while it's happening, or some woman is looking adoringly at him, or....
"What we need is the Magical Armour of Goshienkel!"
"Quick, what's that over there?"
"It's the Magical Armour of Goshienkel! And it's just my size!"