TheFork users who ate in Merate have said really good things about Made in Sud, Lab di Revolution Food & Music and Al Berso'. Maybe you should give them a try!
Some of the more popular Pizza restaurants having a terrace in Merate according to TheFork users include:
Lab di Revolution Food & Music, with a 8.5 rating
Oasi 2 , with a 8 rating
Beefour, with a 8.9 rating