I wouldn't recommend mod podge bc it's super water soluble, if the pony ever got wet it'd all melt off.
I use duraclear matte varnish and it's, like, fine. Sometimes it dries pretty freakin glossy, other times its a nice satin. But it's pretty permanent and waterproof. So that's good! I also use a couple Rust-Oleum top coats in gloss and Dead Flat and they've worked ok so far! I'd love to hear other people's answers for this though, always looking for new recommendations