Earlier in their conversation, Cherry and Ponyboy both discover that they are extremely like-minded and share the commonality of enjoying sunsets. Despite all of this, Cherry confesses at the end of their conversation that she probably will not be able to acknowledge their new friendship in the halls at school because of their socio-economic differences.
Ponyboy validates Cherry's concern, not wanting to cause any more difficulty for her socially, but his final statement of "Just don't forget some of us watch sunsets too" reminds Cherry that greasers are not all that different from the Socs; rich or poor, both can see the sun set from their side of town. Even though they may not be able to be openly friendly at school, Ponyboy and Cherry can remain friends through their shared love of sunsets.
Ponyboy means that the Greasers and Socs really aren't so different from one another. They feel like they live in two different worlds, but Ponyboy is reminding Cherry that theire worlds aren't as different as they seem. The Greasers and Socs don't live on different planets;no group is an alien species to the other. People are just people, no matter their race, wealth, or social status.
He means that the greasers and the socs are not that different from one another, despite their economic and social differences - that the sunsets are the same on the poorer side of town, and that the greasers may have more to them than being the "tough" kids.