I too do not believe in the platitude that 'everyone is loveable' but I do believe that everyone could be loved by someone ... history is full of 'monsters' that were deeply loved by someone. The worn out cliché 'you have to love yourself first in order for other to love you' always seemed poorly worded to me. There is no shortage of people who love themselves first and foremost ... to the extent of narcissisms. I think the original idea meant to be conveyed by that saying was/is to be open to receive love of another. Because when we do not 'love' ourselves - we are closed to receive, or even to recognise the love of another human. Either romantic or not. These are just my thoughts.
This is how I see things.
I do not believe being loved by someone has any relation to a person's "actual" worth or value or really says anything about them. It is merely the circumstances of life that some people are surrounded by adoring masses while others are bereft of any real and true warmth and compassion. I have seen many psychopaths, narcissists, and related malefactors with no shortage of romantic suitors or "friends" who they are willing to take advantage of, or who will even take advantage of them. I do not see these people as "loveable", but they are loved by someone. They may have something to offer someone, which is what the other party seeks, and that fact exists independent of their character or worthiness.
Many relationships, romantic, or platonic, are illusiary and not based on love. Either person in question will disappear if they feel the other person does not have anything to offer them. They would have no qualms stabbing each other in the back if they had something to gain from it. There are many people who are in relationships only to fulfill some need or deficit within themselves, and who the other person actually is is irrelvant to fulfilling the role of "romantic partner", etc. True or unconditional love is the rarest thing there it is. Few people are willing to put up with someone who does not offer something to them, whatever that might be, for better or worse.
As with a lot of things, it is a possibility for anyone, but that does not mean it will actually happen.