Hi,
Under full load it is very possibile and acceptable to a transformer being
around 70-80 degree Celsius.
Without thermometer you can judge that with your finger if you can hold your
finger on the transformer
only for a short period of time, that going to be around 70-80 degree
Celsius. I would consider this is a normal operating tempreture at full
rated load.
Cheers,
PeterS