Unfortunately MEMS accelerometers are not very good performers and if you integrate acceleration expecting to get velocity you'll find there's a lot of drift, so that after a certain length of time the results become meaningless - basically MEMS accelerometers are no good for dead-reckoning (unlike expensive aviation-grade accelerometers for instance). However amateur rocket flights are short-duration so you probably get something usable, but not very accurate.
Its best to combine multiple sensors to allow for one to drift-correct another - for instance if you use barometer or even GPS altitude data to correct the velocity drift you should see much better data overall - this idea is called sensor fusion.
More direct measurement of velocity can be done with a pitot tube and pressure sensor, these days that's probably fairly cheap and compact to do.