The sensor X shows the reading Y. Is this normal?

If you want to find out whether a sensor value is within normal limits, select the menu item Window > Show Overview (or press ⌘ + 1) and look for the limits in the third and fourth columns of the sensor table. They contain the known official limits Apple has specified for readings acquired at the given sensor locations. If no limit is given in the table, this either means that it does not make sense to specify a limit for this particular sensor (e.g. the battery capacity should not have limits), or Apple hasn’t published any specifications for this sensor.

Note that all up-to-date Macintosh models automatically shut down or enter sleep mode if one of the readings exceeds a critical limit. For this reason it is impossible that your computer can be damaged in an over-temperature condition.

Further information: Displaying Sensor Data