He's not really a 'tank' in the same category as poppy sion or malph/mao, he can facetank stuff for his team but you're kind of like other bruisers in which you take damage by becoming a big threat and killing the adc if you can. I max Q in lane and if its a tough lane just farm with cleavers and you should really never fall behind in CS. My order is Q-E-Q-W-Q-R. Max Q after your 1pt in E max W.

Is basically my core, you can swap sunfire/visage order on lane. Top it off with thornmail if you like it, I generally go for randuin or mallet because its really sticky and helps to stay on target and adds some more damage. If its a ton of magic damage you can grab warmogs or GA. Even with swifties nerf being fast is super important and burning agony/swiftness mastery should give sufficient tenacity. Its a judgement call if AD's are getting out of control and you think tabi is the better buy.
Even if you come out a wee bit behind if you choose to trade, cleaver/E/grasp adds alot of damage and your passive helps you regen back up faster. I tend to think his laning phase is fine, if you get a cleaver chain you can just pile on a ton of damage. Through multiple trades there should come a point where you can ult and eventually push them out or just kill them.
Runes I run hybrid pen reds, since W/grasp/Q/sunfire are magic and you have beefy autos giving you hefty mixed damage. Health/lvl yellows because I like having the bonus scaling health, you can run armor if you like. Blues I run scaling magic res/lvl because it adds up and besides visage you really dont have alot of good MR/health choices. Quints I run armor to beef up the early game, it ends up being 13 so its actually a little more than armor yellows .
Masteries you want grasp, its good. Runic armor, swiftness. Everything else in that tree is up to you, I prefer the summoner cd but if you want the lane sustain be my guest. Runic armor is very important because of the health refunding on killing things with Q. As far as I know it works with that plus your big natural regeneration making it have insane value with mundo. I grab offense for the ability damage, feast because its not alot but its some more extra safety and the spellvamp because natural talent is pretty worthless. Oppressor has good synergy with your output because of cleavers.
I believe you need titanic to be that 'threat' as in , this dude is gunna kill my carry I better do something about it kind of guy. Q- AA/E-AA-Titanic is alot of fat upfront damage. Sounds like you just need to play the lane more I think its actually pretty fine. You're not exactly set up with a play making kit though.
I played mundo almost strictly through the platinum brackets (top lane) so I personally don't think he's terrible. I'm d5 scrub right now with a 70% winrate over a span of 60+ games on him in ranked. Just to show you it is possible to carry with him