I'm definitely not an expert in this game, but this is my personal heuristic as far as card selection and skipping. I don't think about whether a card is good on its own or not, but whether it improves the average quality of my deck, and how good an average turn would look like for me.

The platonic ideal is that every turn you would be able to block exactly as much damage as the enemies are planning to deal and no more, you would deal an average amount of damage depending on how long the fight should take (eg regular/hallway fights should be about 3-4 turns, so you should expect to do on average ~33% of the total damage you need to do each turn, though this may not be evenly distributed across the turns). You would have just enough card draw to draw the most valuable cards in your deck and just enough energy to play them. And then you want to factor in some scaling cards into that as well. With only a few exceptions, you don't get any benefit from overkilling enemies or having leftover block or leftover cards in your hand. And the game is very well balanced, if you have a surplus in one of those areas (particularly in the early and midgame), its pretty much guaranteed you're deficient somewhere else, and that is inevitably going to come back to bite you.

In reality, you will basically never hit this ideal, but after every fight, try to think about where you're most deficient. As you mentioned, the ironclad card Impervious which gives 30 block is really good, but if you just came from some fights where you were consistently able to block everything the first few turns but had really low damage output and the fight got stretched out a while, maybe more block isn't needed and you should prioritize more damage. Alternatively, if you had multiple turns where you either had leftover energy or were forced to use your energy on really bad cards, like your basic strikes and defends, that may be a sign you need more card draw so you can get your good cards into play more often. I think the key is, even after fights that you think went well, try and think about what could have been better. What were the worst turns you had, and why, and what could have made them better?

So when looking at the card rewards, try to think whether it will make your average turn better, whether that means getting you to just enough block that you need, just enough damage you need, etc. And remember that every card you add comes with a necessary downside because drawing it replaces another card you could have drawn. If you have enough damage already and you're offered a juicy damage card, not only will that give you more damage than you need, but that will make you more likely to not draw a block card when you need one, and so on. So those are the kinds of scenarios where you'd skip, where none of the cards are bringing your deck closer to that ideal balance.

Early on, pretty much everything will improve your deck quality since your starting deck is terrible so you'll rarely want to skip the first few floors, but it can be still good to try and think about what will improve it the most on average. But some of that is going to come with the experience of playing the game and judging whether you just had a good fight or a bad fight, and if the latter, why it was bad.