Is it true that generally chicken ham is not as tasty as pork ham?
I don't know that one's particularly 'tastier' than the other, but it's more about what you're familiar with.  It wasn't what you were expecting, and so it tastes bad to you.

For example, say you're at a restaurant, and you order a soft drink, but they bring you something that looks right, but isn't.  (eg, you order root beer, and they bring you a cola or visa versa).  Even if you would normally like what they brought you, your mind is expecting something other than what your taste buds get, and it just tastes wrong no matter how much you typically like the drink they brought.

It's similar to trying foods for the first time -- you often need to try it multiple times until you can really taste it, because your mind isn't sending you signals of 'that wasn't what I was expecting' and 'I don't know what that is, it might be poisonous or have gone off, I should puke this up right now'

There are lots of low quality pork-based hams, so it's impossible to say that all chicken hams are worse than pork hams.  But it's worth noting that cured porks are high in glutamates, which we sense as meaty (aka umami, as it's not all from meat).  If the chicken ham doesn't have MSG or some other form of added glutamates, or if they try to make it 'healthy' and reduce the sodium, it's going to taste 'flat' as compared to pork hams.

For the time being, I'd recommend that you ask for extra tomatoes (high in glutamates) and extra salt or other seasonings until your taste buds are more familiar with the chicken ham.  If you can handle dairy, also switch to the parmesan bread when it's available.  Your other option is to avoid ham entirely for a few months so that your memory of what that sandiwch normally tastes like isn't so strong.