Would it be frowned upon to wear your ACUs to Arlington National Cemetery, just for tourism and not for a funeral?

There are two trains of thought here. If you want to wear your uniform due so within compliance to the Uniform and Grooming standard. That covers the "can you" wear it. You wouldn't be the only servicemember wearing your uniform to the Cemetery. Next to the "do you need to wear it". It isn't required and nobody would judge you if you don't. If you are in DC as a tourist there are a few things to think about. Don't draw attention to yourself if you don't need to. It's not as dicey as DC once was but it is still a major city, tourist destination and tourists get targeted as easy targets. It's also more comfortable and easier to navigate the city wearing street clothes. Then there is the case of "should you". Just know that once you put on that uniform in that setting you are automatically associated with everyone in the Military who work ANC. It's your call. Unless you have a specific purpose behind it I would recommend opting for something comfortable. Tourist life in DC is hot, sweaty, and requires lots of walking.

PFC (Join to see) I have added topics to your post that will flag knowledgeable folks that can provide the correct response. AR 670-1, which covers the Wear and Appearance of Army Uniform and Insignia, is the reference procedure.

A couple of choices here realizing that Arlington is hallowed ground.
-Wear civilian clothes. Be a respectful tourist. I've done this.
-Wear the prescribed uniform for military members working in the District of Columbia. This may take some searching, but there are rules.