A shame this developer didn't actually add in anything new that's refreshing. It's just Summer Memories++ or Summer Memories, but with a winter skin; a winter mod alternatively. He got lazy and kept his bad decisions from his previous game in his new one. The sequel curse strikes again!
I'm curious, what mistakes?
The game does indeed feel like an expansion, but given the nature of the first game, it really always would. Narratively (as much as there is) and relationship-wise, it's natural progression in that it does really take everything to the next level and builds upon the previous game in ways that make sense, while still adding plenty of new content, especially for the side characters which didn't get all that much attention before. There are also new mechanics and additions to the make.
Reaaaaally not sure what the heck you were expecting, but as far as sequels go, this not only well within reasonable expectations, especially for a pron game, but also makes sense given the nature and narrative of the first game. Combined with the previous games, you can clearly see improvement from title to title too, and heck, if I made something that blew up, I would definitely want to make more of that thing, and given everyone's excitement for the title, that was a smart choice.
Even small stuff like the controls have been improved a bit, so yeah... Winter Memories seems like a perfectly fine sequel to me nor describe it as being victim to the sequel curse, which you also seem to misunderstand.
The "sequel curse" is usually reference a sequel that seems to utterly miss the point of what made the media prior to it so special, very often because the only motivation was repeating the money instead of realizing the enjoyment is what led to the money.
That's kinda the opposite of what Winter Memories has apparently done.