TheFork users who ate in Lille have said really good things about La Table de Sam, Mar y Sol and Africana Meltingpot. Maybe you should give them a try!
Some of the more popular Tapas restaurants having a terrace in Lille according to TheFork users include:
La Table de Sam, with a 9.4 rating
Mar y Sol, with a 9.1 rating
Africana Meltingpot, with a 7.6 rating