I2C read of De0 nano accelerometer

Yes you are right, this means the slave is acknowledging properly. But if in your application you need to know if the slave answered properly without having to use SignalTap, it could be a good idea to read this value in your code and change an output pin such as "SlaveOK" or "SlaveError" depending on the results.

The next thing to check is that you are reading the correct register, and indep check if the slave is doing any clock stretching. For that you can change your code so that the clock output is set to 'z' instead of '1' (you do have external pull-ups on both the I2C clock and data, don't you?) and see with signaltap if the slave is keeping scl low for any reason.