Would Mexicans get mad if Americans went to Mexico and waved the American flag on July 4th?

No. IMO that's what makes America so great a melting pot. A place where immigrants of different cultures or races form an integrated society: ♣

Not really, you see Mexicans fall within the consideration for "Americans", since America is a very large continent and includes countries such as Canada, Peru, Venezuela, Cuba, Chile, Mexico, Argentina, Brazil etc..

Unfortunately (or fortunately) there is no official "American" flag, as of today, and therefore no one would recognize an invented flag being waved by a person from somewhere in this continent.

I dont think the Mexicans would care the least,hence many of their families in the states(Mexican Americans)wave the American flag with alot of cultural pride.

No it would not bother them, many people are buying real estate in mexco now & are moving there full time.

you should try it and then tell us how it went =)