For me, I think it is in top 5 toplaner right now for sure. New Mundo became a very great scaling tank. He got a great passive that works very well when somebody is ganking you. In order for the enemy team to succesfully gank you, they need at least 2 cc abilities. If they don't have it, feel free to overextend and use your ghost when necessary.
For me, each game I play, I deal the most damage with mundo. And I think that is understandable since you have a lot of abilities that deal damage. So dont be scared to fight, you have damage and health to fight versus the enemy team.
Also, Mundo has a range of different abilities that you have to get used to, it is not easy at the beginning to understand how to play with mundo at his maximum. It will take you a while to get used to the way his playstyle is. You can be aggro and defensive but these two options are not easy to get good at. Practice makes you better!

When playing this champion, use the most of your abilities. Look where your passive goes and follow it. Dont try to just run into enemy team without anyone behind you, since you will die eventually from their damage and cc. In lane, try to get as much minions as you possibly can, since the gold from 20 minions is equal approximately to 1 kill so don't try to go for kills and play rather safer. Wait for ganks from jungle and don't be shy to ask for them since for mundo, it is important to have a bit of a lead at the start. After that you will anyways come up as a solid strong tank that can engage well and run forward to the enemy adc.

Main parts you have to focus on your lane are:

Farming minions
Looking at the map
Trading enemy toplaner with your abilities
Ask for ganks
Engage in teamfights


Farming minions - it is important to take a good amount of minions while laning since the gold that is coming from minions is useful. If you outfarm enemy laner by a decent amount, you will be way stronger than him.

Looking at the map - always keep an eye on the map, it will help you to understand how the game goes and where you teammates are. If you see enemy jungler taking herald you can simply roam to him and scare him off or even kill.

Trading enemy toplaner with abilities - you can trade enemy with your q. If you hit it, you will deal some solid damage that will help you in case jungler will be involved in your laning. Also try to poke the enemy with your e when you kill a minion with it. It does not have huge range but still it is possible to hit it, if you are trained.

Asking for ganks - this is fairly important thing that you should do when playing Mundo. You can get a good lead with the help of your jungler. If you are doing fine and other lanes are not so good, then ask your jungler to focus more on botlane and midlane.

Engage in teamfights - as I have mentioned before about looking at map, when you are laning and you see that there are teamfight signs in the topside jungle, you should follow your team and get there as quick as you can.

In teamfights, you should focus enemy adc at first. This will help your team to get rid of annoying damage that comes from the back. It will also reduce incoming damage to your allies and will greatly increase chances of winning a fight.

Try to tank as much damage as you can and protect your team from enemy abilities. This is important, because you will probably be the main tank in your team, which means that everyone else will try to deal damage. You should really try to prevent your teammates getting hit by the enemy spells.

Also consider focusing enemy midlane, if it someone that has not much mobility and is dealing a lot of damage. Would be great if you try to engage at the really front so your team will feel safe and will follow you in the fight! :D

Better top wins!