League of Legends (LoL) Question: Are heal viable as a Mid laner?

Heal is pretty common on poke and support mages in mid lane because it makes them very difficult to kill. It's pretty much just a better form of Barrier unless your lane opponent has easily applicable healing reduction outside of Ignite (like Fizz). Even then, Heal still gives a movement speed boost so it's not bad in any situation.

Kill/burst-oriented mid laners like LeBlanc still usually take Ignite. Leblanc is pretty safe as it is, so running another defensive summoner on her is imo a bit of a waste. You'll want Ignite to prevent your target from easily surviving your burst with Heal.

A few mid laners take Teleport, but usually only for specific champions or if you expect to get poked out of lane rather than bursted. TP mid isn't too common but it's good.

heal like PsiGuard said makes you harder to kill. I like the fact that Heal, despite it's recent nerf is still really good with its 30% MS boost and healing reduction effects are removed. it could get you out of some really sticky situations like(defensive) giving you that last bit of health denying your enemy from bursting you down and with the ms boost you could run off to safety. or (offensively) you could use it like a mini ghost to close the gap and launch your finishing blow to your opponent saving your (imo) much more valuable Flash for other purposes.

Although, I would never place heal on all the champions I use in the mid lane.
it all falls down to your playstyle and what you are comfortable with.

try it out on champions you like to play with. then see if your gameplay improves
then try it out on champions you don't seem to excel in. it might open up new mechanics for you.