My first impression of Clunk was that he was just designed to be a beginner's character and I found him too boring to use for a long time, but I've been using him a lot more recently.
Self-destruct is useful against gullible players but overall I found it to be too much of a money sink to be worth it. As you find better teams, downtime (from getting killed or going back to heal) becomes a much bigger issue, and 'splody Clunks are usually dead weight between SDs. If you invest in bite instead, the only time you have to teleport is usually for upgrades.
Push turrets ? His damage is ridiculously low.
Not sure where you're getting this impression unless you're using SD for turrets. His missiles do full damage against turrets, which is 14 right out of the gate. They outrange most of the attacks coming from defenders, and if they step forward to reach you, Clunk's bite can reach most of the way through the turret.
Defend turrets ? Lonestar and Yuri are light years above him for this.
Lonestar isn't a good comparison since he's just flat out broken. Still, Clunk's missile range makes him able to retaliate without constantly getting hit through the turret by explosions or the bull. Yuri can defend well with mines, but Clunk's health lets him just march through several of them. If he's one of those immobile laser spammers, the threat he poses doesn't make up for him acting as a turret accessory instead of a team member.
Attack in 1vs1 ? He gets dominated by Froggy at any level, by Lonestar once he gets his 3 dynamites and missiles, and by Leon and even Voltar end-game. Also, he can't reach Yuri.
Early game, Froggy can't do enough damage to Clunk to win 1v1 unless you're just sitting still and shooting (or worse, SD spamming).
Lonestar, as already mentioned, outclasses everyone at nearly everything, but Clunk also has easy access to homing missiles and excellent mid air control.
Leon's going to be dangerous to most players in 1v1, but he loses a lot of effectiveness in the air.
Voltar's also going to be lethal to anyone late game, his drones are ridiculous. Snare bite can prevent him from floating back and forth above your team, and SD can wipe out all his drones at once (although he'll usually spawn more right away).
If you can't reach Yuri, he's almost always in a spot where he isn't a threat to you. Yuri is my main, and matches where the other team jumps up and down desperately trying to hit me are usually the ones where I end up with a K/D of 15/0 or better, while my team shreds their turrets unopposed.
Attack in team ? He slows his partner and make them an easy target.
They're not chained to Clunk, they can still evade just as well as they could on their own. Unless Clunk doesn't bother with the speed upgrade, slowing the team down is a minor issue, and I always find a slow team more effective than having everyone recklessly charge ahead. Clunk is also one of the best (if not the best) at pinning other players.
Some other points on Clunk:
-If you bite creeps, you get the lifesteal recovery on top of the usual 30 health.
-I originally shrugged off homing missiles thinking they were training wheels, but now they're the first missile upgrade I get. Not going to bother listing all the advantages, just try them a few times.
-Clunk is usually at an advantage in the air. He'll last a lot longer there than he will if he sits in one spot waiting for SD to cool down. This applies to turrets as well, he can attack from long range and force the turret to turn farther to track him, or drop down from above, shoot, and fly back up before the turret can aim at him.
-Once your missiles are upgraded, if you're not fighting you can fire them to cycle through and have your best one ready.
-You can move faster while shooting by hitting the attack button for each shot instead of holding it down all the time.
-The upgraded fourth missile is an exploding/AoE attack.