I don't think its crazy, you'll just want to work around the heat in some areas
Why should the heat be a problem? During day you are driving. And you'll have AC in your car. In the morning and evening temperatures are comfortable.
Its a problem to the extent that you have to work around it if you want to be active outdoors - not every day naturally works out to have activities planned in the morning and driving during the day. The number of people who have miserable and/or dangerous outdoor experiences in the desert attest to the fact that, at the very least, its a factor to consider while planning.
Moab can be pretty hot - no?
Moab and the immediate surrounds can indeed be stinkin' hot in summer. You wouldn't want to be doing long hikes out on the slickrock during the middle of the day. The upside of Moab in summer - its a great time for river-based activities, and Moab has a great big river right in the middle of it. But other things you'd want to choose carefully - get out in the early morning, when you'll beat both the crowds and the heat. Afternoons are for swimming in the river, or less strenuous activities at higher elevations (like around Dead Horse State Park or Canyonlands Island in the Sky). There are also a good number of canyon hikes that have water and shade, even some swimming holes, also a good choice in the afternoon.
In general, summer is a double-edged sword in the canyon country. Pre-monsoon summer is the ideal time for slot canyon hiking, but can be oppressively hot and dry out on the slickrocks.
Grand Canyon rims are fine summer - the crowds are much more of a problem than the climate. Hiking very far below the rim gets unpleasant pretty fast though. Like others have mentioned, you might consider the North Rim. Its higher and cooler and less hustle-bustle, although lodging is tight unless you plan on camping. There are also many better options for hiking through forest to secluded rim viewpoints, a much more agreeable activity in summer than the inner canyon.