Yes. I'm sure a rough looking game is going to look better on a OLED relative to a TFT screen but you're missing my point. Launch games are meant to WOW and demonstrate a leap in graphics etc... Not look better purely by having a better screen.
Given a chance, every game will want to wow people and demonstrate a leap in graphics.
Launch games are tight in schedule, and will be surpassed by the next iteration. The raw hardware should be able to show the leap relatively easily. Even if they play the same PSP games, Vita version should be noticeably superior.
Besides dual sticks and OLED screen, Vita's PSN features are much more fleshed out, better than PS3 even. Web browser is also better than the PSP and PS3 ones.
Vita's challenge is not performance. It needs to stand out from iOS/Android, not PSP.