Skill level necessary to clear HW MSQ content

[MistakeNot]

That is absolutely contributing to your issues, if not being the entire cause of them.

i90 class-quest armour is well below your expected minimum for Heavensward.

Your MSQ quests will be rewarding you with i115 HQ gear. This is your bare minimum starter level for equipment.

ilvl 90-100 is perfectly fine when starting HW content. It is just barely enough for the top-level ARR content. (Minimum ilvl to be able to even enter the most difficult ARR content is i90)

As noted you will get i115 HQ gear as quest rewards from the early quests in HW, so you are obviously not expected to have better than that when starting.
As you get further into HW content you will need to upgrade your gear though. By the time you do the level 51 MSQ dungeon you should preferrably have at least i115 gear.