No, see the course notes (https://forum.duolingo.com/comment/17638579).
Do/Naddo is only used in the simple past tense (in the wild, in some areas people do sometimes use it more widely in other past tenses).
Here 'Yes/No' would be Ydw/Nac ydw when answering (or if answering as 'I' to a singular chi question).
It would be Ydyn/Nac ydyn if answering as 'we' to a plural chi question.
"If my answer was no here, would it be? Naddo."
I got the mushroom question to translate in the lesson on "taswn", "basai", etc. which made me wonder how I would ask Cynphony's question in Welsh, but I don't think I have come across one quite like this before, because 'was' in that sentence is copula. Would I have to say something like, "Tasai fy ateb ydy 'No' yma, fasai hi ydy 'Naddo'"?
You might ask:
- Taswn i eisiau ateb 'No' yma, faswn i'n defnyddio 'Naddo'? - If I wanted to answer 'No' here, would I use 'Naddo'?
- Na fasech, dim ond yn y gorffennol syml mae 'Do/Naddo' yn cael eu defnyddio yn yr iaith safonol. - No, Do/Naddo are only used in the simple past in the standard language.