It can or it can be a sign of an on-going lameness. If you are seeing a chronic lameness you need to have a good exam performed and talk to your vet about options. Leaving certain things untreated can allow for faster progression.
Yes, club feet cause lameness.
A clubbed foot it a contracted tendon, meaning that the leg with the club will not be able to reach as far. The un-evenness in the stride and musculature will cause lameness because generally there is more stress placed on the club. The actual hoof will also be a size or two smaller than the other, and a different shape. This imbalance also leads to damage resulting in lameness.
I do not suggest ever buying a horse with a club foot unless it is for very light work. The horse will most likely not be able to compete or put up with strains of being ridden for long periods of time in hard work, or on uneven ground.
The best way to tell how bad the club is and how much damage has already been done is to build a small ramp about the size of the horse's hoof that is raised about 1/4" or 1/2" in the back. Stand the horse's club foot on this for 30 seconds or 1 minute and then immediately trot away on solid ground. A sound horse will show no visible lameness, a clubbed horse will trot off with a varying degree of lameness dependig on the severity of the club.
A less severe club can be managed by farrier work and stratgic conditioning. More severe clubs generally pasture-bind the horse.
I have heard that taking a horse with a club foot out for long endurance type rides on varying terrain and the horse will naturally stretch its tendons back out to where they should be. I guy from riding club swore that was how he cured his horse from club foot. He then also changed where he was boarding his horse to an open paddock where the horse could run 24/7.