yeah...as long as it's not a directory that serves a specific purpose to the system you should be fine.

In unix-like systems there are some fixed location where some things are supposed to be. The reason for that is making the filesystem consistent on every machine and thus ease the work for developers. This is called Filesyste Hierarchy Standard, you can find details here: http://www.pathname.com/fhs/
Even if you probably won't get hurt mounting a disk under, e.g., /data or /mp3, an hypotethical utility that deals with user-mounted fs might not behave correctly.
The best thing to do is mounting the actual fs under a subdir of /mnt, and symlink it to /data, if you find it handy.
or if the actual content is in folder mp3 on disk
Symlinks are treated as the file/dir they point to, except some cases the most important of which is:
*most* programs, when run in recursive mode will not follow symlinks
This way you effectively have your /mp3/Dylan directory without violating the FHS, since your actual data will be under /mnt/mp3