Well, if the mempool is congested like it is now, will it take longer to mine the block too ?
No. The average time it takes to mine blocks depends on the difficulty and the the total hash power of miners. The number of transactions in the mempool has nothing to do with the block time.
It may worth mentioning that, the average block time is always approximately 10 minutes on average and the difficulty is adjusted every 2016 blocks to that time close to 10 minutes.