No, thats not always the case. I have a beard and have been dressed in "Sunnah attire" before (which I do wear from time to time) and have met or been talking to people and they still didn't have a clue that I was a Muslim. Not that I'm out there trying to prove to anyone who I am, which is not what those t-shirts are about if that's what you're thinking.
My question to you is what exactly is "Sunnah attire" anyway? Is it some cultural style of clothing worn by a relatively specific group of Muslims in a certain country. Is it something that was only worn by the Prophet (saws) and his companions. Or is it any type/style of clothing that fulfills the requirements of dress (such as making sure certain parts are covered) as set by Allah and His messenger (saws). I mean if I go to one part of the world I will see Muslims wearing clothing like this (shalwar kameez)
and yet if I go to another part of the world it might look more like this (thobe)
or what about this one
or this shirt
with these pants which are above the ankles
those last two outfits pretty much just looks like a normal dress shirt and some pants....so is that still "Sunnah attire"? Also a lot of the Hadiths that I have seen talking about the Prophets (saws) clothing mention that he wore Izaars. Now I'm sure there are some brothers out there that wear an Izaar but I have yet to meet one.
My point here is that I don't think Allah is going to care what type/style of clothing we are wearing as long as it does the job of covering our bodies in the correct way. So why do some people get so bent out of shape when I wear a different style of clothing which is still doing the job correctly. Let me guess your going to say "because it's imitating the kufar", right? Well now days there really isn't much that you could wear that isn't being worn by some kufar somewhere in the world. For instance, some Christian women (such as the Coptic Christians) cover their heads with a scarf that looks almost identical to the hijab. Some non-Muslim men have beards too. I have seen some Hindu women who wear clothing that looks just like the shalwar kameez. There are also some non-mulsim youth in my country who have started wearing this little hats called "du rags" (or something like that) that looks just like some Kufis, and infact the Kufis that I wear have been mistaken for them by non-muslims. So it looks like there isn't much we can do that couldn't be compared to what the kufar are doing. I would also like to refer you to this:
What is the ruling on using clothing of the kuffar (unbelievers)?
The use of clothing of the kuffar (unbelievers) which they have worn is permissible as long as there is no known najasa (impurity) regarding it, because the default for things in general is tahaarah (purity) and it is not overridden by mere doubt. Likewise, what they have woven and dyed and what has been imported from their countries are also permissible, as long as they do not comprise anything haram (forbidden), because the Prophet (peace be upon him) and his sahaba (his companions) wore items the kuffar had woven and made, and likewise the Prophet (peace be upon him) wore a Roman jubba (a long outer garment, open in front, with wide sleeves).
Sheikh Muhammed Salih Al-Munajjid (www.islam-qa.com
I'm not saying that there is something wrong with wearing so called "Sunnah attire", as I myself wear it on occasion, all I'm saying is let's try and be a little more accepting and not look down on brothers and sisters who choose to not wear clothing from somebody elses culture.