Your mandarin will taste great even if it is not true to seed. If you can plant it outside do so in a hill about 5 foot diameter and 1 foot tall. You will get fruit in 4-10 years depending on weather and how you feed the tree.

Most citrus grow true from seed... this includes almost any sweet orange, as well as key limes, grapefruit, calamondin, most tangerines and tangelo. Varieties that will not typically come true from seed include the temple orange, some kumquats, Persian lime, pomelo, Meyer lemon and a few others. It all gets rather complicated. I'm not entirely sure about Clementines, but I don't think they do either.

But anyway, the downside for those that DO come true is the REALLY long juvenile period most citrus need before they are big enough and old enough to actually bloom and bear fruit. Mandarins can take 6 to 12 years, depending on growing conditions. By that time the plants will be quite large.

If you have it planted in the ground in a mild climate and can let it grow big, you will probably get fruit sooner rather than later. If you are growing it in a pot in a northern climate you may never see fruit, or you may give up on it and throw it out before you do! So this is why most people purchase grafted plants. They can usually bloom and bear right away.

Grafting means you have to 'attach' a proven, known bearing variety (the scion) onto some kind of rootstock. I suppose if you have access to a bearing mandarin, you could graft it onto your seedling. But otherwise, you'll have to wait for your seedling to get old enough.

I thought hybridized citrus don't come true.

It depends less upon whether they are hybrids and more on whether they are capable of polyembryonic development (producing multiple clones within the seed) and to what degree.