Lala's inventions in To-Love-Ru are always defective, mostly the cause of problems in every episodes. But why are Lala's inventions defective when Peke once explained (Motto To-Love-Ru Episode 6 "Night Tutor" to be exact) about how universally genius Lala is?
Looking at the list of Lala's invention on Wikipedia page of To-LOVE-ru - Japanese version), or a less complete list on To-LOVE-ru Wikia, most of Lala's invention has caused some sort of trouble as seen in the source material.
- There are a number of inventions which are created and used right away without testing.
For example, unnamed drug in chapter 54 of original series, given to Rito when he disguises as his father during Mikan's house visit.
- There are a number of inventions which explodes, or activates too easily, or not idiot-proof.
- There are a number of inventions which has unwanted side-effects.
For example, Pyon-Pyon Warp-kun usually only warps living things, leaving the person naked after warped.
However, there are a number of inventions which are mostly stable, such as Peke, D-Dial and the expansion to Rito's house.
My theory is that all of Lala's inventions are work-in-progress or proof-of-concept, and the more an invention is used (which is equivalent to testing), the more stable it is. She is a genius in the sense that she can quickly create a working proof-of-concept. However, since she is not making a product, she doesn't feel a need to make her inventions practical, safe-to-use and idiot-proof.
According to Wikipedia:
Although Lala appears to be somewhat air-headed, she is well known on Deviluke for her genius-level intellect and enjoys making all manners of inventions, often prompting some disastrous effect or another.
As far as I recall, the defects in Lala's inventions tend to be side-effects rather than anything else. For instance, personal teleportation device being an undressing personal teleportation device instead. So it seems coherent to me with her being depicted as both an airhead and a genius.