Are average daily temperatures affected by DST?
They compensate for DST.

That is, for any daily measurement, they use the same universal time every day of the year.  If they take a measurement at 7:00 local time in summer, they will take the same measurement at 6:00 in winter.

(So if you do see a DST effect in the data, that would be due to traffic and other local anthropogenic effects shifting by an hour relative to the time of measurement!  Perhaps due to urban heat island effect, or forcing by anthropogenic aerosols from traffic or industry.)