I've heard that applying for the Green Card lottery is a huge red flag in the eyes of American embassy officials and should be avoided if one plans to visit the US in the future.
Is there any official confirmation for such rumors?
The US Government doesn't publicly specify how it decides on a person's eligibility for a visa, hence there can be no "official" answer to your query.
In general, your local embassy will only know that you applied for the Green Card Lottery, if you are selected as those names are sent to the embassy, non-selected names that are not. But they do ask if you have previously applied. And so it is up to you to decide on how honest to be.
What an immigration lawyer told me (a neighbor) is that if your qualifications are solid for the visa, having applied for the lottery has little effect. But if you are borderline, then it becomes a bigger factor.