Does the foam from instant coffee always taste bitter?
With proper coffee the foam is referred to as crema. It is typically quite bitter itself (as coffee tends to be anyway), which seems to be a function of the emulsion of the aromatic oils  in the crema and CO2 from the bubbles. I don't know that the crema on an instant coffee is any more bitter than the crema on an espresso
Instant coffee aims to replicate the crema to give the appearance of a quality coffee. I have found (I'm from NZ, where we drink quite a lot of instant coffee generally), that instant coffee tends to have a faint chemical taste in the crema that isn't present in an espresso coffee. I suspect this is a result of the process of manufacturing instant coffee, or even perhaps as a result of aiming to get a crema on the instant without the pressurized equipment used to make an espresso coffee