One of my favorite comfy outfits as of late has been a tank top with a pair of baggy basketball shorts. And right now, I could care less if anyone thinks it looks weird haha. You try being sick constantly and carrying a baby in your stomach and see if you wanna be comfy ;)
I wear tank tops with baggy shorts all the time but, I think that you should get a guy's opinion even though I have never got a complaint!
It wouldn't look weird, it just might make her look like she isn't into guys. If you're wearing a tank pair it with shorter shorts so it doesn't look so guy-ish like a dude wearing a wife beater and shorts.