number-one
Explanation
In this comic, a child is confronting their father, angrily asking something like "What the hell is this? You think I'm stupid?" and demanding attention -- apparently upset that the father doesn't pay enough attention or prioritize the child. The father, looking disheveled and tired, tries to placate the child: "Dad is... okay, okay, I'll get you a telescope or something."
The final panel shows a coffee mug that reads "NOT SIGNIFICANTLY DIFFERENT FROM A GOOD DAD" -- a statistics joke presented as if it were one of those "#1 Dad" mugs.
The humor works on multiple levels. First, it's a parody of the ubiquitous "#1 Dad" mug, replacing the superlative with the language of statistical hypothesis testing. "Not significantly different from a good dad" means that, statistically speaking, you cannot reject the null hypothesis that this dad is good -- but it conspicuously does NOT say he IS a good dad. It's the most lukewarm endorsement possible, framed in academic jargon. The comic depicts exactly the kind of mediocre-but-technically-adequate parenting that would earn such a mug: a dad who is distracted and half-hearted but ultimately caves and tries to do something nice. This is a characteristic SMBC joke combining statistics humor with family dynamics.