People don’t like Mako because he was involved in a lot of the romance issues that the show had in Book 1 and 2, they blame him for it all, ignoring any mistakes that Korra and Asami made with regards to the romance issues. Mako made mistakes too, but it is a more even thing between the 3.
I personally really like him as a character, but feel he was let down by the writers by having a lot of his focus revolve around the romance and not a massive amount about him as a person. He had some great moments, but he needed more than just a few moments here and there. I think many have calmed down on him because of Book 3 and 4 not featuring him as heavily, but he definitely gets a lot of hate, unjustfied for the most part IMO.
For me I tend to focus on the positives rather than the negatives so for me Mako is one of the most intelligent members of Team Avatar, he is very calm and cool-headed which is needed in a group led by Korra. He can be very tactical with plans while she is more of a blunt force approach type of person, he is a great detective/police officer so he has valuable skills for the group.
He is the best bender in Team Avatar after Korra, probably the best Firebender in Korra, he gets some very strong wins in fights like against Ming Hua and taking out the colossus. He is even praised by Amon for being an impressive bender. He can generate and redirect lightning, great all round firebender. With the intellect to use this power effectively.
He can be a good leader if called upon, he was the Fire Ferrets captain after all and I would assume next in line to be Chief of Police after Lin.
He is loyal and will put himself on the line for those he cares about.
I think so many miss out on these points about Mako because of such silly reasons.