We recently ripped out our deck because it was rotted and laid a level of cinder blocks 10ft x 32in for a small patio, basically just a step for coming out the back door. I was just wondering if I could use mortar the better level at a slight angle and maybe cover with polyurethane.
No. Mortar isn't designed to carry span loads, and your block patio will almost certainly sag and settle, meaning your mortar will disintegrate in short order. Proper concrete would even need to be poured at a thickness adequate to span gaps, say 3".
To my knowledge, polyurethane isn't an appropriate masonry sealer. There are other products for that purpose.