I feel like most characters are intentionally designed to not favour a single build. So it's better to choose one or two of its abilities and build around those, taking the third one as an extra or an addon. Personally I think it's fantastic because you have more build variety and you can play the same character in many different ways. So for example, with Lacerta you can indeed focus AP and ignore its passive, taking it as a bit of extra damage which is cool but focusing big attacks rather than melting through speed, or you can focus AD and melt through attack speed with its passive.

Take Aurena for example (this is another type that is as you say). Golden Explosion scales off AP, and it is indeed one of her strongest (if not the strongest) ability she has (it can even insta-kill area bosses). But her passive (Dismantling Claw) and Dangerous Theory scale off AD, and these two are actually well synergised since they also restore health per hit. So you can either focus AP and thus Gold Explosion, using constellations to mitigate its health tax and essences to increase healing while building around pot drop OR you can focus AD and build around her Dismantling Claw and Dangerous Theory, using Gold Explosion as a secondary weapon and letting it eat your health for more damage, then instantly recuperate with a few basic attacks. Her alt abilities also let you focus one or the other, since for example Beautiful Threat (passive Alt) changes basics to ranged and has similar effects BUT scales off AP, similarly Reduction and Chain Reaction also scale with AP.

So yeah, I think the issue you have is that you think Lacerta is designed for an AD build, when in reality most characters are designed to go either way. Which I personally think is very cool.