Powering up Alolan Pokemons

Alolan Exeggutor is a better Exeggutor in its role as a Grass Attacker, as the added Dragon typing helps Alolan Exeggutor resist the water type attacks you're bound to receive while using its Bullet Seed Solar Beam moveset. Barring the community day exclusive Venusaurs, A* Exeggutor is a top grass contender. So yes, power that up.

Your only hesitation should be to consider whether now is the most relevant time.

Sure Milotic is a Water type and a great defender, but the walking wall and general added hassle of evolving a feebas may be a deterrent to seeing them frequently to matter for you to have a Grass attacker. In short, A* exeggutor was great to have during the Kyogre/Water Week, not so much now.

I can't speak for the other two. Would need to do more research.

Alolan Eggy is a good attacker. He double resists Water moves and he is only second to Frenzy Plant Venusaur but than again he has to compete also with Raikou and Zapdos. As Dragon he is useless.
Defender: I dont see him there. Being weak to Dragon is a big minus and a double weakness is never good (Ice might not be that common for attackers but still)

A Muk:
For an Attacker he doesnt have the stats. As Defender he has this weird combo. He is weak only to ground on the other hand he doesnt resist much and doesnt force a Machamp to swap out.

A Golem: has a moustache and is cool I guess...
Double rock Ttar here and some good rock and electro attackers comming Gen 4. Doubt he has a use. As defender he is solid I think. Double weak to ground is ok since it is not really used much.

Conclusion:
- all are good
- none is a must have
- A. Eggy is the best of the 3
- A. Golem has a moustache

Exeggutor is the best of them, a very solid grass attacker with great TDO against many targets. Not necessary if you have a team of FP Venusaurs already.

Golem is the next best, it's a solid rock attacker like Kantonian Golem and a mediocre electric attacker. Not worth powering up as a rock if you already have SD Tyranitars or normal Golems and not worth powering up as an electric if you already have Jolteons or legendaries.

Muk is the worst as it doesn't really have a role where it's good at. Ok defender but it lacks what makes Kantonian Muk good, resistance against Machamp. No real applications as an attacker.

The only Alolan with better performance than its Kantonian counterparts is Alolan Exeggutor if use against water type.
Like everyone said, double resist to water.
Others, not so much. Cool to have one though

Alola Exeggutor: slightly less DPS than normal Exeggutor, but more bulk against Water. As a defender, it has a double weakness to just re-released Articuno, so not very good.

Alola Muk: a solid attacker, but losing resistance to Fairy and Fighting is a downside. As a defender, it is only weak to Ground, which is good since Dark Pulse can deal some damage to it before Muk faints.

Alola Golem: tank counter (and still does very good damage) against Ho-oh, with a double resistance against Flying. As a defender, Groudon is just here.

Alolan Golem loses the double weakness to Grass and Water, trading them for a double weakness to Ground (a much weaker attacking type). This makes Alolan Golem much better suited for gym defense than regular Golem, although still nowhere near as good as Blisschanlax. (IMO a double weakness makes almost anything unsuited as a defender, aside from this and regular Eggy's double weakness to Bug attacks.)

Alolan Eggy is quite good against water, gaining a double resistance over regular Eggy's normal resistance, for the price of a few points of attack. Competitive against water on sunny days when it's boosted, still good unless it's raining, when electric is the best.

Alolan Muk is really pretty, and if you get gunk shot, it throws a garbage can for it's charged attack, which is kind of fun. Adequate defender, otherwise not overly useful.