Dollar Tree can't be beat for things like cleaning supplies, commodity-type CPG items like soap and seasonal decorations-- but those items likely aren't where they make their money (they average a 30% GM, imported trinkets will have higher margin but lower volume than consumer staples). I've spoken to managers at multiple locations and the consensus top category by volume is candy, followed by paper goods. After that, the responses tend to vary.
Things like food are already a bad deal when compared to standard retailers-- they are typically smaller quantities at a much higher price per unit, and the products they carry are more processed so as to reduce costs and increase shelf-life. Then when a 1:1 comparison is made, they are often 30-50% more expensive (e.g. can of chunk light tuna for $1 vs $0.65 or less at Aldi). Going the route of 99 Center or the fast growing Five Below chain where they sell items for under $3 or $5, but with better quality items would have been an interesting approach, but that probably encroaches too much on their "up-market" Family Dollar locations? All being told, I wonder the price elasticity of candy and hyperprocessed foods are.