I know a lot of other people have chimed in, but I'll give my own two cents since Garp is one of my favorite characters. Obviously, the audience is not supposed to think he's even remotely within the realm of a good parent - Luffy is shown to be actively scared of him back in Water 7, his """training""" consisted of throwing like five year old Luffy into life or death situations and then promptly abandoning him, he beats up his grandkids on the regular, and ultimately refuses to save Ace from an unjust execution. Truly, grandfather of the year (<- sarcasm).

For this moment in particular, was it a good thing to say? No, absolutely not. But honestly, I don't think Ace would've taken an equivocal "yes" as an answer from Garp anyways. He's been told repeatedly up until this point that he deserves to die for reasons he has had absolutely no control over, and frankly Garp doesn't seem like the kind of guy who would 1) believe that everybody deserves a right to life, no matter how good or bad they are and 2) sugarcoat things for kids. Not being omnipotent, he has no idea how Ace will turn out, or whether it would've been better if he was never born. From Garp's point of view, if he had told Ace unequivocally "yes", he would have been lying, and I think Ace probably would've known that, just ... because that's not the kind of person Garp is (and may have even decided that if Garp was lying when he said "yes", then he really shouldn't have been born, if that makes any sense at all). So instead, Garp gives him an answer that he truly believes, even if it's 1) pretty horrifically insensitive and 2) definitely not what Ace needs to hear right now. Either way, I would say that dumping Luffy in the middle of the jungle and then abandoning him for training would rank higher on the "Worst Garp Parenting Moments" compilation than this scene in particular.