It is fairly difficult, but there are a few reasons why I found the difficulty more pleasant:

• pacing: the most difficult parts you'll first encounter are optional bonus objectives (strawberries) that you can skip and come back to later. So you can take your time and get them as you go when you feel like it, and if you get frustrated you can continue with the main storyline for a bit (this happened to me a few times when I was playing, so it was basically continuous enjoyment)
• very forgiving: in Hollow Knight, it's like Dark Souls where if you die, you lose your in-game currency and need to make your way back to your corpse to recover it, and if you die again then you lose it completely. But in Celeste, you get a checkpoint at every "screen", so it's very forgiving to experiment new techniques and just keep trying over and over again. There are some longer patches so that it doesn't feel too easy, but overall I really liked it
• most of the storyline is accessible without having to do much extra. There are some more challenging levels at the end that require finding some hidden unlocks earlier in the game, and beating some tougher challenges that I'm going through now.

But fair point, even the base storyline is quite challenging, especially if you don't love this genre. I'll edit my comment to be more clear. They do add some "assist mode" to make it easier if you want to enjoy the story, but I'm not sure if it's still fun to play it that way.