Well not in DMC5 it's not, that's for sure.
I play and enjoy lots of character action games, and grades certainly aren't the be-all and end-all of the experience. I'll strive towards not getting shit grades, sure, but I won't worry too much if I don't 100% SSS/PP the whole game. If a particular encounter ranks me down for not having a high enough combo, I'm not going to spend time figuring out what it wants. I'll just move on.
The real thrill of character action for me comes from efficiently utilising available tools, and overcoming difficult enemies quickly, and coming through unscathed- not necessarily memorising endless superfluous pretty moves to jam into a jump cancel exhibition for a grade. DMC5 simply doesn't scratch my itch. DMC1 is still basically the perfect character action game- enemies are fast and ferocious, but through skill, knowledge, and practice, the player can decimate enemies like they're nothing. It's the ultimate glass cannon experience.
With each DMC title, they've flirted with style/combos and placed increasing importance on it, but with DMC5 they go too far. In lieu of any real challenge until DMD, the combo experience is all it has. And unless you know about the boss trick, you've gotta go through a very easy game twice to get there. No major character action game fails like this (but as pointed out by another GAFfer, it's an inevitable consequence of a AAA DMC game in 2019 combined with Capcom's nervousness about staying true to the series).
And that's why it's boring.