In the social context you said it in? Probably.
In the context of what people on this site would want you to say? NTA.

It's sort of the meme. You're "correct" but in order to understand that, you more or less need to be terminally online (how does commas work in English?).
People in your social circle will perceive you as an asshole, despite understanding your reasoning.

Is one an asshole for making fun of a categorically larger asshole? Based on the principle of diffusion in asshole thermodynamics, the only logical answer is no.

Fuck the troops

NTA, keep on keeping on

lol

LMAO imagine dying in the most unbalanced war since like...the Anglo-Zulu wars

special kinda suck

Yes, YTA. The dude deserved it but I'd never voice that to a family member for no reason. And no, you later discovering that he made it up doesn't change anything, you're still the asshole because you had no clue they made it up at the time.

ESH: The guy for trying to get clout for having a dead troop relative

you for replying "womp womp" instead of "Hopefully he died a hero by pulling pins on multiple grenades in a barracks in the middle of the night"

b a s e d

Following edit: Even less no

AITA

womp womp

SHUT UP SHUT UP SHUT UP SHUT UP!

NTA by circumstance, but YTA by behavior, I guess. In context it's actually funny but I think being mean about dead relatives (esp ones that would be closer, it's not his second-cousin-twice-removed-on-his-dad's side, it's literally a parent's brother). You can ask sympathetically-framed follow-ups ("Gosh {{Dislikeable Liar}}, I'm so sorry about that! What branch was your uncle serving in?") to to make them show their ass if you don't believe them, and then let them demonstrate them being an asshole, but unsympathetic-to-neutral bystanders might conclude that you were (albeit slightly, and in a way that attempts to be lighthearted). Don't worry about it though, if this guy sucks everyone knows. Ultimately you understand the context.

If you dislike them then why do you care?

On the grand scale: no, you're just acknowledging someone who was doing bad shit as part of a bad system got what was coming for them in a karma sense.

On an interpersonal scale: generally mocking the death of a family member is frowned upon by that person's family. so if they didn't already dislike you back, they probably do now.

Sometimes the "right" thing doesn't need to be said, if you know what I mean. And saying things, even if true, has a right and wrony timing that will determine if you're the asshole in life.

Also, in context, did this person know why you said womp womp? As in did they know you were saying like "he was doing shit. I don't care."? or did they have no context and so assumed you were just mocking the death of their uncle generally just to be mean. If they don't know your politics, which are not all that common in the US (assuming US due to this situation), or they're most braindeads who never think like "gee, why was my uncle in a foreign country across the world?" I have to assume they're either a chud who thinks that him dying there is heroic or someone who is mostly neutral but finds mocking their uncle distasteful. Most Americans probably very genuinely seriously would not understand (until it's explained then maybe they get it- but people seriously go without thinking about shit their entire lives) why the uncle deserves, to whatever degree, mockery and definitely not reverence. People can't even grasp why Palestinians might fight and kill Israelis. The Iraqis are pretty much similarly dehumanized to US but it's US soliders getting killed, so there's another layer to digging past the BS "they're terrorists!" stuff.

So perhaps you should womp womp and then when they get pissed you lecture and educate them on imperialism. One day, perhaps, you will womp womp their uncle together.