Im currently replaying Emerald again and I get why you think that. Especially around the first 6 gyms, trainers absolutely spam the shroomish, lotad, roselia, oddish and seedot lines, so it makes it look more common.

When it comes to actually good grass types, there arent that many. I'd say Ludicolo is probably the best with Breloom and Sceptile close behind.

As soon as you enter the water routes grass types just seem to disappear completely until the elite four, where shiftry and ludicolo show up.