Howdy, neighbor. Fancy seeing you here.

There's a save point in the room
right before her. It's hardly unfair, and as much as it might suck clearing the area and then dying to her, it isn't the game's fault you went and fiddled with something unknown without saving first.
She's also not really a superboss--no there are simply no earlygame weapons able to do enough damage. Once you have better gear, she's a pushover.
I do, however, agree that there should be some kind of indication further interaction leads to a boss battle--but not because of it being difficult or inescapable, for the above reason.
There are many other interactable statues (all of which have their own unique appearances, so it's not as if hers being facing away from you is enough indication) in the game that exist to give information, and you encounter several prior to the Count's mansion. This establishes a precedent that interactable statues give information that either describe the world lore or provide a piece to a later puzzle. Considering there is one such puzzle just a few floors above her statue, this precedent is doubly implied.
So for her statue to, by all appearances, suggest to the player it's the same as the others, and then lead to a fight no earlygame file is going to beat, then I think some other indicator needs to be there. Normally I'd say something like "make her statue glow all evil-like," but the same problem of established precedents exist, as by that point in the game linearity there isn't anything else enough to establish that "this glow means bad stuff, be careful."
Perhaps a prompt something to the effect of "An uneasy aura flows around this statue. Touch it? Y/N" or whatever.
To reiterate--I think there should be some secondary confirmation or indication of her statue being a boss (which at that point in the game nobody's got a chance of killing) due to the violation of the established gameplay precedents involving interactives and statue busts, not due to the outlandish difficulty spike of her fight at that point in the game.
EDIT:
Having thought more on it, I think I can understand why it exists in its current state: to scare the player, evoke fear, that sort of thing. But I don't think it does this as effectively as it could.
Of course, everyone's different and processes things differently, but ignoring for a moment the gameplay precedent issue mentioned above:
For me, stuff I can't hope to kill yet popping out of nowhere is not what ever made Castlevania scary for me growing up playing it. It was the building of tension in brutal stages that I knew all too well would culminate in some crazy awesome new boss I didn't know anything about yet. It was about killing one tough enemy with only a sliver of health left and praying there wasn't a second one the next screen over. It was evading one Medusa Heard over a pit and not knowing if the next one was coming high or low.
Now, the statue woman doesn't build anything like this. There's no substance to the fear factor. It's not unlike cheap jumpscares in B-grade horror films, actually. Now, she doesn't need a whole level building up to her, since she shows up at its start and that's obviously not practical. But there are other ways to build tension and suspense--the Book of Caged Souls later on is a prime example of this.
First time I found it, I read its description, felt uneasy about what would happen if I said yes and opened it but also anxious to see what it contained. THAT is the kind of thing that made Castlevania scary for me. Not knowing what could happen next and having all the freedom of choice to make that leap or run away.
The statue ghost being early-game unbeatable is a great way to encourage a precedent for backtracking with new equipment later on, and a good reminder that it's always handy to pick up that stronger new weapon at the shop as soon as you can. But her current state does not build the fear and shock value I believe she's intended to have. She just comes out of nowhere like a bad jumpscare, and instead of fear or anxiety when encountering her the first time, I had a moment of "wait, what the fuck? Why am I in this new room now?" followed by an impatience and annoyance to get killed and the fight over with once I realized I wasn't gonna beat her.
If she was meant to scare, then at least for me she dropped the ball in how she does so. Good horror conveyance is not about a scary untouchable thing showing up out of nowhere--it's hearing it thump around downstairs, or being told of its horrible nature in hushed whispers or scrawled testaments in an old and forgotten page in a dusty book, or catching a glimpse of it lumbering down the far hall as its breathing echos all the way down to where you're standing. The demons and what-ifs conjured up by the uncertain mind are more terrifying than any horrible monster jumping out at you from nowhere. If she had the same prompt as that Book later on, I would have doubtlessly had the same reaction of cautiousness mixed with anxiety, and THAT would have made her pocket dimension and imposing fight convey to me elements of fear.
But she does have potential. I merely think that her execution is somewhat lacking for what I'm fairly certain is her intended purpose.