You'd think it would be relatively easy to believe the MC is another world summon since he's male, yet he can use magic. He also doesn't behave like a man of that world would, which should strenghten the more concrete evidence of being able to use magic. Above all, it is a world of magic, so they shouldn't find the idea so impossible. In our universe it would sound stupid, but that's because we don't have magic.
It's no surprise they didn't care much about otherworlder. We may not had magic but we had theories of times, spaces, and matters. It's like a girl having a male dong
(dick if somehow someone didn't catch it) and claiming she is from another world we wouldn't believe her, we might categorize her as a mutated human with 2 reproductive organs and go on a day or play along with her theories of another world.
If they can bring some strange knowledge with definitive proof and is very ground breaking in today's science advancement he could be labeled as genius like Einstein and the others but still will not compare to the idea of another world.
or some really severely religious fucker up there would call them an apostle of god or something and demand the city to listen to the supposed apostle's word of enlightenment and develop the city with the knowledge bestowed upon them that would require a massive change to the very basic of the country's system of operation and society as a whole with promised (unproofen) potential
Only if they can bring some mysterious power like magic with no limit and a big fat proof of evidence (like showing them magic and such), or something akin to being a vampire/mimic in world full of myth of them with no fully explainable and solid evidence of existence, or also breaking the limit of today's human growth potential by a big factor will they believe they are from another world.