Is there anything to do at Mekar Island?
Currently, there does not appear to be anything unique on this island. It is simply a reference to the sage Makar from the Wind Waker (according to the Wiki).
There are a few other sites that discuss the island, none of which have found anything interesting:
Besides the enemies found at night here, there is nothing else to the island. According to the Reddit source above, this is also a possible reference to the very first dungeon in the original Zelda game:
I think it's a reference to the first ever Zelda dungeon, Level 1. There's a dead tree, and four piles of bones in place of the surrounding four smaller trees
Here is a screenshot of what they mean:
Image source from linked Reddit above.
There is also a video of someone exploring the island at night. They take out all the enemies, and walk around some, but don't find anything unusual otherwise.
I don't know if this is specific to Mekar Island, but you can also cut the grass. This is easier to do with a one handed sword, and through this it's possible to find about 5 fairies, which are useful for healing potions. However, I haven't found anything unique, like little people with korok seeds or anything, but it would be interesting if there was a secret that everyone was missing. So far, I cant see one myself. :)
Mekar Island is actually a hiding spot for some faeries. If you have fewer than 4 faeries and you start cutting grass on the island, you should be able to find enough to get you back up to 4-5.