There is a lot to like about Agnes Baker. However, the Heirloom of Hyperborea is not one of those things. In fact, I'd say it's the weakest investigator-specific card in the game.
Think of it this way: you have to spend an action to put the Heirloom into play. It costs three resources. And when you play it, you're giving up a card. How many cards does it have to draw to start before it generates real value above and beyond that 3-resource, 1-action, 1-card investment? I would say about six. Are you really going to play six spells over the course of the game? Even as Agnes, I would say that's a tall order. Bear in mind that several Agnes staples like Drawn to the Flame and Forbidden Knowledge do NOT have the spell keyword.
Moreover, playing the Heirloom will massively slow you down in the early game. Agnes has more strong assets she needs to get into play early than any other investigator. There's Shrivelling, of course, and Agnes really wants Peter Sylvestre as well. She probably needs Rite of Seeking or Flashlight to do some investigation, and lots of Agnes builds use further things like Fire Axe and Ritual Candles, among others. Can she really afford to invest critical early-game time and resources into a card that might, if everything breaks right, turn a very small profit over the course of the game? (But more probably will end up in the red?)
The Heirloom's effect only gets worse if it's drawn later in the game. If it's not played in the first couple turns, it has basically zero chance of paying back its cost.
It also eats up an Accessory slot, which is inconvenient because Holy Rosary is typically more powerful for Agnes.
Of course, the Heirloom is by no means a crippling thing for Agnes to have in her deck. She just uses it for +2 to a Strength or Will test. But all the other investigator-specific cards are at least occasionally worth actually playing. (Daisy's is a little borderline, but you can imagine scenarios where it would be worthwhile.) Agnes's is basically only skill test fodder.