I totally agree with all these things--she's slow, she's fragile, and she's mana-intensive. However, I think she has a lot higher of an upside than a planeswalker. Sure, she costs mana, but she's essentially reads "Once per turn cycle, cast any [Human] from your library." She doesn't charge a premium the way most tutors do, and she doesn't lose you any card advantage. Actual creatures tend to be a lot stronger than a 3/3 Beast token. My fear is that, like
Arcane Savant, she's not going to do the fair, fun things that I'd like to see her do. There's not nearly as much potential for abuse, but it puts an additional constraint on every Human you add.
By way of comparison, is
Stoneforge Mystic +
Batterskull too strong for your cube? That has about the same speed as Lin, and I'd say that the upside is (very) roughly the same. Lin can then go on to grab more cards and has a much wider range of targets.
(Lethargic Ninja Edit: Stoneforge
does tutor on ETB, which may make Lin "more fair" than SFM/changes the reliability of this combination. I totally forgot about this, and it makes my comparison somewhat less valid.)
And all this is without considering the second ability. Yeah, it's crappy on its own, but it means that you can tutor for your Humans over and over again. It's never going to happen in a million years, but I had a nightmare last night about this and
Avalanche Riders (actually, I wouldn't mind losing to that, and I'd like to think I'd even enjoy it).
I'm very curious as to why you're more concerned about her grabbing cheap creatures. Would you mind expanding a bit more on that point? I agree that Mom is a very strong creature to grab, but a defensive one, and, considering the limited lifespan of Lin as a Bolt-On-Sight priority, I'd rather grab another threat.
Of note, you make a good point about "being a removal check" not necessarily being a downside. I agree. No matter how flat your power band, there will be things that are removal checks, and, if you have multiple things that are removal checks, something is going to be the biggest removal check out of them. We just have to make sure we're happy with what it is. For me, I prefer my removal checks to be payoffs rather than enablers because it's much more satisfying to
Vindicate a
Griselbrand than a
Coffin Queen.