A few things:
-You don't have to be a decent actor to play a hero in a cbm. Just look at RDJ, Chris Hemsworth or that guy who played QS in AoU (man was he bad). So her not being a good actor shouldn't even be an issue. If she auditions and does a good job: give it to her. Jai Courtney still gets roles and he seems to be doing a good job in Suicide Squad.
-We got Gal Gadot as WW, she can't act for shit either but killed it during her audition (so I read). So yeah. Same thing could happen.
That being said, I've never seen her in anything so yeah...
RDJ has been acting since forever. And he has chops. The Iron Man role isn't going to strain him any, he could probably do it in his sleep.
Thor is a wooden character. He is very straightforward, there isn't much nuisance to him. I think they got an actor to match that.
When Gal Gadot says a line in a movie, I go with it. She isn't some world class actor, but she doesn't stand out. Rousey so far has stood out. It's exactly what you would expect when you put a person that has never acted before in a movie. I mean we can shit on actors for being in bad mlvoes or having dumb lines, but a lot of them have been doing it for years. Plays and short films when they were younger tons of auditions and projects before they made it big. It's a skill.
What's the point in having to give someone acting lessons on a multimillion dollar project when there are hundreds of actors that could just deliver the line well enough. I don't even know how much her fighting experience would come into play with a movie like this. They'd only let her do so much for insurance purposes, and I'd bet a lot of it would be CG anyway.