Tipping is a non-issue for the Poles and most Europeans, and tipping drivers is a completely alien concept altogether. Restaurants and free walking tours - yes, drivers no. No local would do that.
However. Everybody has heard about or was subjected to the crazy tipping culture in the US so tipping from the Americans may have become expected. I've read a few posts on other forums over that last two or so years that suggest that the Americans are now being harassed for tips and insulted when they try to do what the Europeans do, i.e. round up or do not tip at all. As soon as they hear American accent, they expect +20%, LOL. That was in the most touristy places in Italy and Greece, though, I've never heard of anything like that in Poland. It looks like it's evolving towards a two-tier system - one for the Americans and another for everybody else. But then, if you don't like it, you have only yourselves to blame. You have been told to not bring your crazy tipping to Europe for decades so now tip generously.
The default face expression in Poland is a neutral one and it's normal to be kind and respectful without smiling all the time. Holding doors for others, giving up seats on public transport for the elderly etc are the norm too so that will not earn you any brownie points either. Basically, unless they do something absolutely outrageous or criminal, foreigners are given the benefit of the doubt even if they do not conform to local norms, so I wouldn't worry about that. If you want to smile, smile.