Can I set up some sort of offer that moves with the price difference?

Let’s say I want to set up a XMR:BTC sell offer - at the time, the ratio is 70:1 so I set up a sell at (e.g.) 71:1 XMR:BTC. But in the event that Monero goes up in value, i.e. to 75:1, I would like my offer to reflect that and go to 76:1.

Can I do this with Bisq or do I need a trading bot?

I’m just wondering because I want to help out and provide liquidity, but I don’t want to lose money in doing so.

Yes, you can make an offer with the price in percentage of the market price.
I think this is the most common way people make offers, fixed price offers seem a bit rare to me.