Hello and welcome to Dakka!
Ultramarines get hate mostly because
GW focus so much of them in the lore, because of Guilliman, and because of a
GW writer by the name of Matt Ward*.
Few people dislike Ultramarine
players, they dislike
GW's approach to the lore. You'll be fine.
*If you want to know more about Matt Ward and why he's a meme in the
40k community, you can google him. Prepare yourself for enormous amounts of vitriol and hyperbole if you choose to do so though.