No, You must enable STMM to have the AUTOMATIC heaps work. You must also have at least two of the heaps set to AUTOMATIC for it to work. It is not generally a good idea to have the bufferpools at AUTOMATIC. A fixed size works good. Set the other heaps to AUTOMATIC.
just a quick example. I setup a new database on a system with 64 GB RAM. The database had a size of roughly 100 MB. There were two bufferpools. 1 for User Data and the IBMDEFAULTBP for system-catalog, Temspace1, ...
The STMM increased IBMDEFAULTBP to 2 GB since the System had enough free memory at the time. Sometimes the STMM ist too ambitious.
I'm running most of the databases with STMM, but sometimes i does not work.
Bufferpools are where DB2 actually works with the data and indexes. If you set them to AUTOMATIC, DB2 will be constantly resizing them trying to get the "optimal" setting. But there is no real optimal setting if they are variable since some other bufferpool or cache will want more memory eventually. Then you have DB2 shrinking or expanding the bufferpools which takes time and will hurt performance. It is better to have DB2 change the sizes of the other caches which are not always being used, but the bufferpools are always used.