Bitcoin mempool has now more numbers of bitcoin transactions it can process per seconds and the only way to overcome it is using more fees to get your transaction quicker. When a transaction remain long in in the mempool, is because the transaction fee they use is less than what the miners are asking for that particular time and it has nothing to do with PEPE tokens and other meme coins, they are shit coins that are independtly on another blockchain like Ethereum and Binance smart chain.
However, we have some meme tokens are ordinals inscription on the bitcoin inputs as metadata. Since bitcoin is an open source, people are allow to create meta data in their transaction as file and this input files are so heavy that you need to pay high fees to get this transaction confirm from miners and you know people love quick returns. This ordinals are the new meme in town, they are been pump with averagely making of 20x, so people are sending in and out of this ordinals with high transactions and it is also affecting ordinary bitcoin transactions, the mempool is full of high fees ordinals transactions that are competiting with transaction confirmation and hence we it is congested.
So when you see PEPE ERC20 pumping, the ordinals also pump as some people see them as meme on Bitcoin but they are not the same, it might serve the same purpose for the meme lovers but they are not the same, one is disturbing the other chains while one is spamming the bitcoin blockchain.