Surprised your agent wasn't able to give you this information.
But on Holland America you will encounter an older crowd and they historically tend to like to dress up a bit more than on some of the other cruise lines. However, Alaskan itineraries tend to be a little less formal than some other itineraries. So, with that said, I've just worn a shirt & tie without a jacket for formal nights and it's been fine. Or I've worn a jacket without a tie and that has been acceptable to. Don't feel like you have to wear formal attire, such as a tuxedo, which is definitely overkill.
But you really do want to go to the main dining room on formal nights as that's when they have the steak, prime rib, or lobster, so you would miss out on a really good meal.
However, should you decide you really don't want to do the main dining room, then there are several other options available. You can do the buffet on the Lido deck, go to one of the specialty restaurants (Pinnacle Grill is one of our all-time favorites), or do room service.