Having your own ideas is a feature of being a researcher, so this is in general terms a positive for your application. A good candidate has ideas, and some sense of what a research project is - formulating a research question, working out what needs to be done to test it, devising a project to fit within the available time/budget/resources.
Of course many applicants don't have a lot of awareness about many of these things (eg whether your idea is good, potential methods, or how much time/budget will be required), and that's fine, you'll learn about that during a PhD.
So getting here at this stage is all positive.
However, it depends on sources of support - both funding and a suitable adviser. Some of those may allow you full freedom (eg a personal scholarship) but others may want you to work on something else (eg an adviser with a funded project that needs somebody to work on it).
Hence you may end up compromising based on opportunities available to you. Having your own ideas is often a positive sign in terms of an application for such a position, but being dogmatic about your ideas (even if they turn out to be of limited interest to others) may not be. It is up to you whether to stick with your idea (but have no adviser or funding, potentially a dead end) or take a position that is interesting but not quite what you first thought of.