Yes, they should be able to fly those flags.
And yes, I should be able to think poorly of them if they're seemingly putting far more focus and love and care towards their ethnic heritage rather than the country they're actually living in.
I'll be fully honest. I have no problem with a mexican or italian or irish or any other heritage individual proudly displaying that heritage. But when your focus is more on that then being American, when you always fly their flag but never the stars and stripes, when you take advantage of the benefits of this country while simply pumping up another, then I've got little respect for you.
However if you knock out Brock Lesnar I'll at least give you a temporary pass. Temporary.