Impermanence (Anicca) and Mindfulness in general Did the Buddha meant to be attentive all the time or is that just an ideal to strive for?
If we are attentive and feel sensations do we always see the impermanence in them? So first to recognize what is going on in- or outside and then seeing the impermanence and potentially the unsatisfactoriness in them, right? But wouldn't that decrease pleasures in certain activities/objects?
That is two questions. I'll answer the second.

Things are not unsatisfactory in themselves because they don't have selves. Anicca is the flip side of Anatta (non-self). Because things are made entirely of other things and aren't independent self-entities if any one 'thing' in the universe changes then everything changes because it all interpenetrates. This is the middle way between existence and non-existence - a big debate in Buddha's time and ever since!

Dukkha (third of the three marks of existence) is what arises when we deny Anicca-Anatta. If we truly see/embrace/understand/know Anicca-Anatta then we are in Nirvana - which is nice not nasty.

So in practice we look deeply and see Anicca-Anatta which feels good. Nothing changes. Ice-cream still tastes yummy but we can see that it, along with our tongue, is impermanent and is also a product of the whole universe. Without impermanence the ice-cream would never have frozen and wouldn't melt. Without non-self the ice-cream couldn't become part of us. i.e. Wholesome pleasures increase in breadth and depth as we see them deeply.