It was indeed used for jousting, it is (like you mentioned) designed in a way that protects the user of wooden pieces which are from the lances when they impact. The Devs of Mordhau could probably tweak it a bit for better usability/realism. For example removing the plate ( or making it smaller), making the helmet smaller in general.
There are many ways to tweak the model for the Frog-mouth Helmet. And personally I think that it would actually be comepletely fine the way it is, because I suggested it because it just looks very cool ( Iām also a giant fan of the helmet and own one myself).
Maybe there could be something added like jousting helmets and eventually a Perk which adds certain bonuses and lets people use jousting equipment. A gamemode could also be combined with that eventually. But these are only ideas and a little suggestion which I think might look cool when used with the right armors and metal tints and what not.