This is not about rules but about fluff.
There are two ways for a follower of Lamashtu to sire fiendish offspring, a mask and a feat.
Would you think it sensible to fluff either a tiefling (for a PC) or a half-fiend (for an NPC) as the product of several generations of breeding involving followers of Lamashtu with either the mask or the feat (or both) with fiendish mates?
Example PC: My parents were Followers of Lamashtu and their unholy rites lead to my being a Tiefling. I managed to escape and are now out to hunt those deranged followers of that mother of beasts.
Example NPC: When the Party finally reaches the high priestess of Lamashtu after fighting through hordes of lesser followers and fiendish animals they find her guarded by her favorite offspring, a half-find Gorilla.
Well, tieflings are at most 1/4 fiend (anything more goes to half fiend). And the offspring created with the feat/mask are half fiends. So yes, repeated generations would result in tieflings. Heck, just two generations count.
Things get stickier if you reintroduce fiend blood in repeated generations- less likely to go tiefling. But you just need a single 1/2 fiend to mate with a new member of the congregation. Then boom- tiefling. Since the general doctrine of lamashtu is about spreading out and being 'fruitful' with the uninitiated... well, it isn't hard to get to tiefling level.
Anything can be a valid offspring of a follower of Lamashtu. Literally.
Half-fiends? Check.
Half-demons? Check.
Half-monstrous humanoid? Check. A lot of the followers are on this categhory already.
Half-dragons? Check.
Half-aberrations? Check.
Half-plant? Check.
Half-anything-that-can-reproduce? Check.
You might need more templates.
Undead is probably one of the few monster types that you would find trouble explaining how it happened. But even those wouldn't be that difficult.
As for tieflings, i don't remember exactly if this was on Demons Revisited or another campaign setting book, but there is a sidebar explaining how half-fiends will very likely have offsprings that are either also half-fiends or tieflings. If they breed with some non-humanoid creature, they will likely spawn another half-fiend, but if they breed with humanoids, the chances are higher of spawning a tiefling than a half-fiend humanoid.