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If I'm writing an actual novel I put grammar and effort in, but with Fanfiction I know it's not actually going anywhere so I don't bother with grammar all that much.
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You already figured this out, but no, it's not a good attitude. Writing fanfiction should really be the same as writing anything else. After all, you have readers that you're trying to net, so you want your writing to be as good as it can be so they can focus on it instead of your mistakes. Clean grammar allows us to enjoy what you're writing. Think of your fic like a road. If the road is smooth, the people riding on it can sit back, look around, and enjoy the scenery. However, mistakes are like potholes. One or two can be noted but otherwise shrugged off for the scenery again. However, a lot of them will make the ride less enjoyable because all we can notice are potholes and bumps all over the place.
That and as one friend of mine put it, if a writer doesn't care enough about their story to clean it up, why should the readers care? For example, you actually outright stated that you didn't really focus on a very basic part of writing because it's fanfiction, so that says to a reader that, first off, you probably didn't focus on making every last detail (even the bits of plot nuances) in the story perfect and that, second off, you aren't actually putting forth your full effort in this fic. Both points tell us that we should probably move on to a different fic written by someone who did put forth all their effort to tell us a story.
No offense, but yeah. Saying that is actually bad advertisement and, well, not that great of a mindset either way. If you're really passionate about writing, you should be exerting all of your efforts and using all of your skills in everything you write, not just the things you're expecting to publish. Otherwise, we can't really give you advice on how to exceed your usual skill limit because we don't know what that limit is.
I was being very shallow. I apologize.