My dream is to make games only for Nintendo and as a result, I'm a huge Nintendo fan. But now starting my career as a game designer, I wonder if I can still rant about bad games publicly and whether its considered unprofessional for someone being a game designer.

It's somewhat inappropriate to negatively "rant" publicly if you are also identifying yourself publicly as a professional in the industry. This is especially true of games or products that your own could be seen as in competition with. Well-reasoned arguments and discussion are fine, and the occasional burst of frustration on Twitter when some bug renders your game un-completable are probably forgivable, but frequently descending into histrionics and bad-mouthing is likely to be viewed unfavorably.

If you're ranting in the sense of "overjoyed adoration of same particular game," then maybe some people will raise a few eyebrows at your fervor but otherwise it shouldn't be an issue.

I wonder if I can still rant about bad games publicly and whether its considered unprofessional for someone being a game designer.

That would almost entirely depend on your content and presentation and to a smaller degree, your sense of self-preservation. The use of the word "rant" though, doesn't bode well.