Are Index funds goal only to match there benchmarks? Shouldn't they try to outperform them?

CYOA
No, the goal of a passive index fund is to match its corresponding index at low cost. Consequently, the TSP funds, with their low tracking errors and very low management fees, are excellent performers. Incidentally, the G fund acts as a stable value fund and, therefore, doesn't have a corresponding index.
The TSP funds are designed to be used as components in a passive portfolio constructed to meet your need for return and tolerance for risk. TSP provides the major asset classes, i.e. cash, bonds, large/mid domestic stocks, small stocks, and international developed market stock.
The L funds represent professionally designed passive portfolios based on the idea that younger investors can assume more risk than older investors. Consequently, TSP not only provides excellent portfolio components, they also suggest appropriate portfolios based on your investment horizon, i.e. time to retirement. Amazingly, the L fund portfolio construction and daily re-balancing are provided for free!
TSP doesn't provide all of the funds that might be desirable. For example, inclusion of REITS, Emerging Markets, and perhaps, international bond funds would be nice. However, as a 401K, TSP is hard to beat. We're lucky to be able to invest in it.

------Jim