I'm not sure how much of a negative connotation "whistleblower" has; that's a question that is probably better asked on English SE. Actually, it turns out there already is a long anwer there on the topic. It concludes that
Today, few people in the United States have any memory of hearing whistleblower used as a pejorative term, and modern dictionaries present it in a broadly sympathetic light.
As far as the Ukraine-related whistleblower, the term is used in no small part because the person in question has claimed and apparently qualifies for protection under section 601 "Intelligence Community Whistleblower Protections" of the Intelligence Authorization Act for Fiscal Year 2014; this section which sets out protections fairly similar to the better known
Whistleblower Protection Act generally applicable to other federal employees, basically prohibiting administrative retaliatory actions against whistleblowers (like demotions etc.) The somewhat misleadingly named Intelligence Community Whistleblower Protection Act is also relevant because it sets out the procedures to use for whistleblowers who blow the whistle on classified issues, although the latter Act doesn't provide any protection for the whistleblower.
My point is that federal law makes widespread use of the term, so it would be hard to avoid in the Ukraine-related matter, even if it has some negative connotation, which I'm not sure it has.
And apparently you've missed the fact that the whistleblower (the first one at least--there are two now) didn't "listen in" on the conversations, but had second-hand knowledge of them.
Furthermore, the act of blowing the whistle, means reporting something that the employee thinks is an illegal practice.
to tell the public or someone in authority about something wrong that you know someone is doing, especially at the place where you work.