I think grown legal adults should date who they want, regardless of age. However a big age difference often means you have no common interests and a hugely different world-view, both of which can be obstacles for a successful relationship.
But if you and this lady (30 = lady or woman, not girl) like each other and get along, good for you both.
I don’t think it’s unusual for a 30 year old woman to ask whether or not marriage is your long-term goal, especially if it’s hers. It’s better for her to find out right away that you are not compatible in that way, than for her invest a lot of time and effort into someone who never plans to commit.
The only thing I’d wonder about is whether she means marriage *right now* which is kind of quick - and weird - from someone you just met.
Be honest with her. Whether you do have a goal of wedding bells, or you don’t want marriage in your future ever, with anyone, be straight with her.