Yes, I agree with you. The default icon is an eyesore to me too. I have long ago changed it. My workaround was to pin Mailspring to the taskbar, right click the Mailspring icon, right click the program ‘Mailspring’ from the popup menu, and then select Properties. Once in the Properties window, change the icon to what you want. I changed it to a variant of the blue Outlook icon. You will still see unread notifications in the corner of the new Mailspring taskbar icon. The only bad thing about this workaround is that it will stop working occasionally (it resets to the default icon), most likely after some major Mailspring updates. But it only takes a few seconds to change and it hasn’t bothered me any. If it did, I would honestly say “Oh man, I can’t take this anymore. It is too much of a bother to keep changing the icon.” But I don’t feel that way. I attached a screenshot to show how it looks on my taskbar. Actually let me show you another screenshot that shows the unread messages. One second. There. That is how it looks when there is an unread message.
PS - To reiterate, I think that the developer should change the Mailspring icon so that it is a darker blue like the Outlook icon in my screenshots. And I think that it should be a blue square envelope instead of a white envelope in a cyan circle. I am just sharing my workaround on what you can do until then.