Is yawning bad for my Cockatiel or not?

Well no it is not that bad, Cockatiels yawn just like we do and they may do a continuous line of yawns which I suppose you'd call it a yawning fit. 

When Cockatiels yawn they usually do this to clear their crops. 

When Cockatiels do excessive yawning it may be a sign that your Cockatiel has a respiratory problem. 

If you think that your Cockatiel has this then seek advise from your avian vet. 

Thanks 

HMC