Asking followers to tag a friend in comments to participate in a giveaway or a contest is considered spammy.
According to the Instagram Promotion Guidelines, "You must not inaccurately tag content or encourage users to inaccurately tag content."
In HypeAuditor reports, comments that consist solely of account mentions — such as those typical of tag-to-win giveaways and contests — are treated as one of several negative signals when evaluating Comments Authenticity. A comment is marked as inauthentic when multiple negative factors are detected at once, meaning a single mention-only comment may not always be flagged, but a pattern of such comments will negatively affect an influencer's Comments Authenticity score.