As other posters commented, in order to file and claim for the type of additional tax relief you are looking for (mortgage interest) you will almst certainly need to submit the full tax declaration form. The simple tax form is not appropriate for that purpose, even with a cover letter. Everything that can be claimed will have a place somewhere on that form, as well as space for explanation if necessary.

I think the best thing to do here is to download the official Neuchatel Tax program (called Clic'n'Tax), fill it in and look at what it calculates your direct tax to be. You can then decide, based on whether this is higher or lower than your current impots a la source, whether or not it's worth filing a declaration. The application may be directly downloaded from:

https://www.ne.ch/autorites/DFS/SCCO...hargement.aspx

It is worth noting however that although the full tax form will allow you to claim things such as lunch and commuting expenses explicitly, the impots a la source tax rate is actually already an adjusted full tax rate which is supposed to already take account of most typical expenses such as these, which have hard cut-offs on the full tax form, so don't get your hopes up.