I know of one person that said just his body heat caused a dip in his memory foam mattress. I'm guessing that's not a normal occurrence.
That is not normal, unless it's a very cheap mattress. They do conform to your body but bounce back. We have one at home that's 15 years old and hasn't degraded at all. Granted, our room does not get to 100 degrees for extended periods of time, but has hit the 90s when the house has been closed up while we've been away. The newer mattresses are much more advanced in their technology. There are forums are available for the various brands that list pros and cons that you can garner info from.