It is clearly important to understand why there is such a big difference in coversion rate. And you should surely ask about it.
You can start by saying "I notice there is a huge gap btw the client's and the competitor's conversion rate, and I want to understand what is the competitor doing differently from us". But don't just stop there and expect the interviewer to give you the answer. You should go on to lay out a structure of where you want to look into and what you want to probe. For example you can then continue to lay out your structure by saying "To undertand that, I want to look into a few areas, e.g. Product, Pricing, Sales process etc. Is the competitor's product any different from ours, and if so, how?...Is the pricing set differently, and if so what's the difference?....Is the sales procoess done differently e.g. our client requires extra steps for a potential customer to convert?..."
By probing and asking the right questions at the enough depth, you will see the differences and see what is the best practice and how far the client is from the best practice. But you need to probe and drive the conversation, not just asking one direct question and then waiting for the interviewer to give you the answer.
Hope it helps.