A trademark isn't "you don't get to use this word because I own it now", it is "you don't get to use this word in this context as it is confusing". If you want to name your restaurant Ludwig, go ahead: you just can't name your filter Ludwig. With the exception of Lo-Fi, where maybe you could make a defense, these names are non-obvious and have never been in common use to describe a set of modifications to photographs. Just because Clarendon isn't a unique word does NOT have ANYTHING to do with whether or not you could trademark it for something.

You can't just say "durrrr... I've heard this word before!" you have to actually show that that word has been connected to that context and isn't some otherwise unique usage, and I simply don't see how you are going to claim that for these words: if you show those filters to people and ask them to describe them, the only reason they would say "Clarendon" is because of Instagram's prior usage carefully associating that word with that filter behavior: if you believe otherwise you have to show THAT, not that the word itself has been uttered by someone in the past.