Index funds, by definition, cannot be a bubble relative to other stocks. Stocks as a whole can be in a bubble, though that is rarely actionable information. What are you going to do, not buy any stocks at all?
Some indices track subsets of the whole market. A BRIC index can bubble, for example, or an REIT index. Or a commodity index. These aren't really index funds, though, they're sector funds that have borrowed the name for marketing purposes while "index fund" is a popular term customers like, like "gluten free" or "free range organic" or "drain the swamp".
The larger question is whether the US total stock market, or the global total stock market, can bubble. Sure. Even these are still "sector funds" when compared to total planetary asset valuations, which are mostly unproductive real estate and short term debt. But now were talking about the collapse of capitalism, not some temporary market blip, and that's not something I really worry about.