Not entirely so. There have already been some developer ports that the WiiU has a few weak spots in it's power (mainly the processor) that make the porting of some games from the 360 and PS3 very hard without decreasing the features. If it isn't worth their hassle, (ie the cost of redoing portions to fit the WiiU's needs) they won't bother with it.
It's probably this. Based on what I've read about it, the Wii-U has a great GPU compared to the 360 and PS3, but the CPU is lacking in comparison. The amount of optimization they'd have to do to make it work on the Wii-U, although possible, probably doesn't seem warranted, especially given the small install base it has currently. They'd probably spend more money making it run comparably to how it will on 360/PS3/PC than they would off of sales on the system.
If a game were made from the ground up for the Wii-U it might be able to outperform the 360 and PS3, but the architecture Nintendo decided on isn't good for straight ports.
Or so I've heard, anyway. I'm no expert, so take what I've said with a grain of salt.