I wouldn't use it for either, but using it for swap-in is not a terrible idea. Whatever LN wallet you're using should already have it's own swap-in mechanism. Usually there will be some screen or address with a label similar to "receive on-chain funds into Lightning". I just use that. Send on-chain, receive off-chain, very simple.

Using Muun for swapping BTC and Lightning might theoretically give you some better privacy just by adding another hop between your on-chain wallet and your LN wallet. Muun will just see the total amount going in and out, rather than individual payments, and your LN wallet will only see that funds come and go through Muun so they won't have your on-chain address.

But for Lightning payments, it simply doesn't make any sense. Why would you want to do on-chain transactions every time you do a Lightning payment? Plus Muun keeps track of your on-chain funds in Muun, so it's not like you get any additional privacy benefits or something.