Question

How would seasons on earth be different if the earth was not tilted on its axis

Answer 1

If the Earth's axis were 'straight' ... pefectly perpendicular to the ecliptic
plane ... then:

-- Day and night would be the same length ... every day of the year,
everywhere on Earth !

-- There wouldn't be any seasons, anywhere.  There might still be some
'weather' ... cloudy days, sunny days, occasional rain, wind etc.  But
there would be no average change during the year.  No hot months or
cold months.  In any one place, the weather would always be generally
the same, every day, all year.  Everywhere all around the equator would be
generally the hottest on Earth, and the local climates would generally get
cooler as you moved away from the equator and toward the poles.