Yes, there are likely some popular ones. Often, these fanfictions gain popularity because they explore the relationship between Mako and Korra in a way that the original show might not have fully delved into. For example, some might focus on the emotional build - up to the kiss, which can be really engaging for fans who are invested in their relationship.
The Legend of Korra comics are definitely worth a read. They capture the essence of the franchise and bring fresh perspectives. The quality of the storytelling and the visuals make them a great addition to the series.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.