Quality is subjective. Everyone has different musical tastes and preferences, and as creators we all have different goals and skill sets. You can't necessarily say that something is objectively bad. But if you're evaluating something against a specific criteria, you can identify ways that it can be improved to reach those criteria.
Are you looking for help on how to achieve your own goals with UTAU covers? Not sure what goals to aim for? Maybe we could describe our personal goals for our own covers, the times that we've struggled, and what we did to overcome those difficulties. Ultimately, it's better to form your own vision for what you want to create than to try and please other people. (Unless, of course, you're being paid to create something on behalf of someone else. Then it's a combination of your own standards and the client's expectations.)