Spending an HTLC output incurs over 400 weight units of transaction data. With the feerates lately being usually in the double digit [s/vB], it would cost you at least 1,000 sats to enforce an HTLC on-chain. So very small payments are not actually added to commitment transactions and this generally applies to any layer-2 technology that is enforced by users’ ability to unilaterally exit. Bitcoin transaction fees are paid in bitcoin and scale with the weight of a transaction, so the issue is independent of the exchange rate.