Since we light up mavilakku for Thiruvenkatavan during Saturdays, as we have to participate in the utsavams we may be advised not to light up lamps during festival time.
Particularly those who stay at Thriumala will not do so as they like to participate in the temple festival. Generally, during utsava time people shorten the thiruvaradhanam for Perumal at home. They postpone the functions like weddings, upanayanam until the conclusion of the temple festival as the festivals are given prime importance.
One can try to participate in Thirumala utsavam and there is no harm in lighting mavilakku at that time. Act as per the advice of the elders of the house.