Yes, it is generally considered impolite and inappropriate to eat sushi with a spoon in Japan or in most traditional sushi-eating cultures. Sushi is traditionally eaten with chopsticks or, on some occasions, with fingers.
Using a spoon to eat sushi can be seen as disrespectful because it is not in line with the cultural norms and traditions associated with sushi consumption. Sushi is an art form in Japan, and there are specific etiquette guidelines that accompany its consumption.
If you are in a formal sushi restaurant or dining with Japanese people, it's best to use chopsticks to eat sushi. If you are not comfortable using chopsticks, using your fingers is acceptable for certain types of sushi, such as nigiri sushi, which is a small piece of fish on top of a small ball of rice.
That said, if you are outside of Japan or in a less formal setting where using a spoon might be more common, people might be more understanding. However, when in doubt, it's best to follow the local customs and traditions when enjoying sushi or any other cuisine.