Yes, this is normal. Linux will use any free memory if needed for cache and buffers - it will be reallocated if needed for anything else (i.e. stating a new program, an existing program needing more memory for something, caching more recently accessed information, or the hypervisor requesting some memory back by inflating the local balloon if you VM is setup that way, and so on).

This is all managed automatically, but if you really want to fully clear it yourself you can by running sync; echo 3 > /proc/sys/vm/drop_caches as root. The only reason I can think of for doing that is if you are running performance tests and want a repeatable "from cold" starting point without completely restarting.

If mysql is using memory mapped files for accessing its data files (which is likely as there can be performance benefits to this method) then any block currently loaded from them will show up as cached data, so as well as the 250Mb input file your ~3G of cache will include things already in memory counted in the "cached" figure before you started, the data and index blocks created as a result of the import, and any related blocks that needed to be read as part of the process.