Ethical? Maybe. Smart? No. Watching out for your best interests? No. (Actually, that last no suggests an ethical question.)
Frankly, it's not up to your Realtor to guess whether a written offer 5% or more below the listing price is "a waste." If you want to offer 20% below, that's your business. She can advise you on how to make the offer appealing. She can advise you on what the comps are.
And she can ask that any buyer's broker agreement between you and her be terminated. If she doesn't want to represent you, that's her right. So, from that standpoint, she can say "no."
But she isn't representing your best interests in declining to make a written offer. That's why her best option might be to terminate the relationship. (Ethically, I'm not permitted to interfere with your relationship with your Realtor, so I can't and wouldn't suggest suggest that you terminate the relationship.)
As a practical matter, offers 5% below the listing price often aren't unreasonable. Certainly not "a waste."
As others have already noted correctly, a verbal offer isn't enforceable. It's not worth the paper it's not printed on, to mangle an old saying.
Beyond that, there's more to an offer than just the price. Seller subsidy, closing date, contingencies for financing or a home inspection, type of financing (ranging from all cash to conventional to FHA to VA), and so on. Many listing agents--if they were to receive a verbal offer--would respond: "Please put that in writing and my client will be glad to consider it."
And finally, unless your Realtor is a mind reader, she has no idea whether an offer will be accepted or not and, thus, whether one is "a waste."
Hope that helps.