Why Ezra is smart she be like she needs a way to fight the dragons what if more come in there no dragon slayers around so she decides to get some dragon slaying armor.
This has absolutely nothing to do with Erza being smart or not, absolutely nothing. Then you might as well say she should have been so intelligent enough to already have such an armour. This has everything to do with practical skill of actually someone being able to make such an armour.
In Monster Hunter Orage the entire series was based around the killing of creatures and using their bodies to make stuff out, so it's not impossible that he would use it in FT, however that doesn't take away that Erza killed numerous monsters, but not a single time has there been a confirmation that she made an armour out of it. Even assuming she would have gotten the idea, then she still would need to find someone who could do it as just making weapons and armour out of a dragon's corpse is not something an average person can do.
And above all even in the hypothetical case she would have been able to create such an armour, it wouldn't be a dragon slaying armour, but just a dragon armour.