Are You a True Batman Fan if You Have Only Seen the Movies?
>Ignoring the 'Batman' part of this question.
Consider this: if you can be a 'true' fan, then you can also be a 'fake' fan. By definition, a 'fake' fan is not actually a fan at all - they're just pretending to be one, maybe to try and make friends or just to be included. However, if you truly like it, regardless of how much you know about the subject matter, you are a 'true' fan. No one can tell you if you're a 'true' fan; only you can honestly say if you like something or not.
Definition of a True Fan
No one knows everything about a character or a series. Not everyone has the time or the inclination to delve into every detail. Some details only matter in a single conversation on a single thread on a single message board that only a handful of people will ever be aware of. It's important to enjoy what you enjoy, without letting others be the gatekeepers of your enjoyment.
Some people feel superior for knowing more about a piece of fiction that’s not to everyone’s taste, but this perceived superiority counts for nothing. Like what you want, don't like what you want. If we were all the same, life would be boring.
My Batman Journey
I've seen all the Batman films (except fan-made ones), watched many different animated versions, and read a handful of comics. I would consider myself a fan of the character and the world in which he exists. However, I can't go into hours of detail about different backstories or numerous characters in the rogues gallery. I have the rest of my life to read about these elements and explore other characters, as well as non-fiction events that I could spend my time on.
Devoted Fans and Their Mentality
Devoted fans, who try and belittle others for not knowing as much about their favorite character as they do, are more likely to be the problem. They focus on minutiae that only matter in niche communities.
The reality is that these devoted fans only know as much as the creators tell them and, therefore, they'll never know everything. Nevertheless, a 'true' fan is simply someone who enjoys the character or series they love, without the need to know every detail.
Conclusion
In the end, being a true Batman fan is about enjoying the character and the world he inhabits. It's not about memorizing every detail, but about appreciating the stories, characters, and emotions that Batman evokes. If you love Batman and the Batman universe, then you're a true fan. Enjoy it and enjoy your journey in the Batman world!