We need challenges
I agree that easy games are funny, killing everyone totally owning the enemies makes you feel like a pro.. exactly, makes you feel, bit doesn't make you!
In everything we do, if something is too simple we will learn nothing, we just show how good we are, on the flip side, if something is too hard we get discouraged and feel a sense of stress.
How to avoid easy games?
If you notice a game being to easy, start to challenge yourself to increase the difficulty, without losing of course.
CSGO example: use lower tier weapons like pistols or a UMP, try new smoke-flashbang throws, and go on different routes.
Overwatch example: switch hero to something that you don't play very well (keeping your role) for example from Soldier to Mccree, or Tracer.
What about hard games?
The only thing I can say about this is, always give your best, even if you have to carry the game, never give up, if you succeed you may enter the flow state (read below), if you lose try to avoid getting mad, losses are part of the game, we always learn from mistakes.
What is flow? from Wikipedia:
In positive psychology, flow, also known as the zone, is the mental state of operation in which a person performing an activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. In essence, flow is characterized by complete absorption in what one does.
How can I enter the flow state?
You enter the flow state when there is a balance between challenges and skills in an activity. You feel a distorted sense of time, feeling so focused on the present that you lose track of time passing.
It's not easy to enter this state but it's totally rewarding, look around the web if you want to know more about this as I'm not the best person the explain it.
Benefits of the flow
In short, really good to improve your gaming skills faster.
Smurfing is the worst thing you can do!
Take for example an elite dancer who has to compete against an ordinary dancer in a competition, after seeing how bad the opponents are, the elite dancer won't even give their best when it's their turn, winning first prize anyway, after seeing how easy it is they will continue to do this in other competitions too - What's the point in this pro competing against a lower tier dancer? To win easily? How can that be satisfying?
The problem in this is, that when the dancer has to compete against others of his or her level, they will surely not perform as he used to do before taking part in the low level competition.
Acceptable Smurfing ways
I will take the Overwatch example only because it is the only one where I can see an acceptable way to smurf.
If you're a main DPS at master rank, and never played as Healer or Tank, but want to improve without having to do it with your main account, where of course if you're master DPS it's possible that you're also a master healer - Then it's good to smurf for 2 reasons;
If you want to improve as Tank, stick with it till the end of the game, don't just switch in order to win a game faster.