Is it important to read the entire prospectus before investing in an ETF?

I would not say it's "important". I would suspect that very, very few people read prospectuses before buying ETFs.

Just like you don't look at the complete list of ingredients or the recipe of a sandwich before you buy it at a restaurant - you just want the high points. What kind of bread does it have and what's in between. Maybe you look at reviews to see if other people enjoyed it (probably not, but it help with the analogy :-) ). You don't need to know how much of each ingredient went into the bread, what parts of the animal the meat come from, etc.

For an ETF - the main points are what sector/style/size it's invested in (or it's high-level investment objective if it's more specific), how has it performed in the past, etc. You don't need to know the exact makeup of investments, its rebalancing methods, its managers, and all of the legal mechanism necessary if things go wrong. It's fine to know those things, but it doesn't give you any kind of advantage. You're going to earn the same returns as every other investor that didn't read the prospectus. It doesn't tell you if the fund is a good investment, what your price target should be, etc.

The prospectus is more important for initial investors to know what they're getting into. After some time has passed, the "proof is in the pudding", and past performance is more important for investors, and the high level objectives can be easily found by looking at the description of the fund.