I've been considering using a Shopify app like PushOwl to send my customers notifications, but don't customers hate receiving notifications?

Anyone you ask will tell you they don't like receiving notifications. It doesn't matter if it's an email, text, ad, etc., we don't like them. That's obvious because our inboxes are full of promotional emails, our social media feeds are full of ads, and our phones keep beeping with new notifications. But, notifications can be beneficial to your customers if you use them correctly. Customers subscribe to receive emails and notifications if they feel they can trust your store will send relevant updates and won't spam them. Customers want to be notified when a product they're interested in is back in stock, or when its price drops because of a sale. For that reason, they are willing to subscribe to the brands they love. As a Shopify merchant, you should use this to increase your sales, and there are great apps, like PushOwl, for example, that help you do that.