It's an interesting theory, and that may have even been the thought process at Nintendo when making each game - Zelda may have been an attempt at appealing to both audiences by averaging the difficulty while Mario had extra levels for players wanting more of a challenge.
However, I don't think they at all succeeded - with very, very few exceptions, every necessary part of Zelda TP is easier for casual gamers than much of what is required of anyone playing Mario Galaxy.
Your description of the difficulty of each is from the perspective of a regular gamer. Imagine how your girlfriend, or father, or even grandmother would approach each game. The difference is that games are difficult for casual gamers for two reasons, while regular gamers only see one dimension of difficulty.
When we play games, we recognize a difficult task as one that requires timing, quick reflexes, or a lot of practice/observation.
When casual gamers play many games, they don't even get to this sort of difficulty. Give your sister a 360 controller and start her on Halo 3's first mission - see whether she dies because she doesn't have the timing down or can't react quickly enough to seeing an enemy or doesn't figure out which guns are good in which situations. None of this will be a problem for her. Instead, she'll die because she can't figure out which buttons do what. Even after some practice, she won't be able to do more than one thing at a time - maybe she can shoot when she wants to, but, when she's shooting, she can't aim or move.
Casuals have a hard time with what we think of as very easy games, but this isn't because they can't do any of the things that a good gamer might pride himself on. They aren't as good at most games simply because they have a hard time dealing with the way games are controlled.
This is why Zelda tends to be easier for casual gamers. Most enemies in Zelda can be beaten with just a few buttons. Z-targeting can even be set to toggle instead of hold, though I've found that most people have no difficulty holding the button - they encounter problems only when they have to be moving their fingers. Anyway, most enemies can be beaten by repeatedly stabbing while the status of all other buttons remains the same. The entire game is like this. Bosses always give you enough time to stand still for a bit in order to first aim, and then fire, the hookshot or whatever at a stationary target.
Mario, on the other hand, typically requires the player to be jumping, while changing direction, and sometimes while spinning occasionally. This is a much more complicated set of activities.
All I've got to offer as backing is some anecdotal evidence, but how's this: of the three casual gamers I know of who have played Zelda (two of them female, for what it's worth), all beat it, and they only rarely died (call it three times or so throughout the game, on average, generally around the first shadow beasts and the ice dungeon miniboss). While they haven't played Mario Galaxy, I know that at least one found Sunshine all but impossible while another thought it was too difficult to be worth playing. None of them have any ability to play Smash Bros, mostly because the most sophisticated strategy available to them is smash attacking while standing still. Only one of them is capable of recovering with any regularity - it requires jumping and then using the characters up-B, in that order, all while holding the direction of the stage. I've yet to see a casual player who was any good at Smash Bros at all, and mostly for these reasons. You can explain to them exactly what needs to be done, and they can understand that, but they can't figure out how to make the character do what they want him to do in the short amount of time they have.
Zelda is much slower paced, and can be played and beaten by seeing it as a progression of one-button tasks. Mario is faster and requires multi-tasking on the controller. That alone makes Mario a much harder game for many casuals.
Edit: To address the obvious objection - that Nintendo already fixed this with the Wii controller - I'd argue that we're not even close to fully intuitive control. From my limited observations, I think that Nintendo's done wonders for casual shooters - aiming and shooting is now achievable for even the least practiced person - but Mario Galaxy can hardly be said to use the motion controls intuitively. It's 'shake to spin'. They would have been better off letting you spin in the air by pressing the jump button again. And the star bits make the whole thing even more complicated.
Edit2: To perhaps clarify the sort of skill I'm talking about, I think that regular gamers eventually instinctualize controller functions to the point that they don't consciously think about which button they have to hit to perform a task. When we play Mario Galaxy, we don't have to go through this process: "An enemy! I want to jump on his head. Which button is jump? A! I'll press A." We think "An enemy! I'll jump on his head." Now what if you have to long-jump, turn 90 degrees, jump, wall-jump, and deal with an enemy? And what if you're rushed because you're on a moving platform or there's a wave of lava approaching or similar. This isn't that dissimilar to a lot of situations in Mario Galaxy, even the bits that you'd expect everyone to see.
Edit3: I'd be very interested in hearing from anyone who knows a casual gamer who found Zelda too difficult while having no real problems with Mario.
Edit4: First, sorry for all the addendums. Reading the above posts, I'd like to distinguish between casual gamers as gamers who only play for brief periods of time and casual gamers as people who almost never play games. I'm talking about the second sort in the above.
As to which one was better, I found Zelda more rewarding. Mario was fun, and a good way to pass time, but I found it ridiculously easy with the exception of one or two levels.
