Nope, very not true. An opioid naive individual will absolutely feel and get nicely high from 20-30mg morphine. 200mg would definitely kill them.
The people that say it's weak are most certainly individuals with HUGE tolerances (or at least higher than your run of the mill opioid user). Anyone who doesn't have a nice tolerance to opioids who says morphine is weak, well, they just don't know what the fuck they're really talking about. Either than or they screwed up trying to smoke/eat(? lol)/inject or plug it, so much so that little got into their system and they incorrectly blame the drug (as opposed to themselves) for their lack of inebriation.
Even with a mild-moderate tolerance to heroin, I used to thoroughly enjoy the mild yet very pleasant buzz of 60mg morphine (and they were ER pills I chewed up no less). Likewise, at the time, 45mg got me a bit more than baseline high. 90mg at the time was a very nice dose too in terms of almost getting to the point of nodding. Of course none of these doses had me FUUUUCKED up and nodding real hard or about to OD, but they absolutely, positively got me high (and as I said at the time I had a mild-moderate heroin tolerance).
So yea, morphine is a wonderful drug %)
btw, if you want to get "fucked up" and experience pretty much (next to the rush from shooting, which I do not suggest you or anyone else get into if they aren't already) all of what opioids tend to offer, 40-50mg would be a good first time oral dose (assuming no tolerance).