When someone is hopelessly devoted to another person, it means that they have given their heart and soul to that person and are totally committed to them. They are willing to do anything for the other person and will always put their needs first. This level of devotion can be both admirable and inspiring, but can also be unhealthy if it leads to codependency or a lack of self-care.
Hopelessly devoted is also sometimes used in a more light-hearted way, simply to express a strong level of affection or admiration. For example, a person might say that they are hopelessly devoted to their favorite actor, band, or sports team. In this context, the phrase is a way of expressing one's fandom or passion.
In conclusion, hopelessly devoted to you is an idiom with a range of meanings, from a deep and emotional commitment to a more light-hearted expression of affection or admiration.