I agree. It's odd because you'd think that with 74 characters they wouldn't feel the need to make earning one character really tough. But they are, a lot of the time. Some are very easy especially if you use a heavy character like DK. But a lot of them are hard. Also, you don't get to choose which character to bring in with you to the challenger fight. It's easy to predict when the challengers will happen (every 3 or 4 fights, I believe someone said it was a 10 min match timer). But still.
What's even worse is that WoL does the same thing. It's very easy 70% of the time, but then you hit a road block where it's very challenging. And because the fighter unlocks are so spaced out, there's basically no easy way to unlock fighters. I guess that's maybe a good thing, spreads out the content of the game. But if it was intended as a learning mechanism, it's actually pretty terrible. I'd rather learn by doing a few matches and steadily increasing the CPU level, rather than having one really hard match after a bunch of easy ones. It is bugging me.