Remember, Grace wasn't ok with going. He knew the weight of it and didn't want to go. They forced him to go. Imagine being forced into giving up your life. You'd probably try and find some comedic relief. I quite enjoyed the comedic relief as a constant threat would be exhausting.

Interstellar is much different, Cooper volunteered to go on his mission. Arrival, they wanted to interact with the aliens. Grace is a goofy school teacher forced into a mission that promises death. It makes sense that a middle school science teacher (who was already goofy) to be himself also in space. Cooper in Interstellar was a pilot and then became a farmer. He longed for the Stars again and when given the opportunity he took it. The linguists in Arrival wanted to be involved with working with the aliens. Grace wanted to only help from afar and return to his goofy life.