Does not doing something mustahabb mean falling into makrooh?

Not doing something mustahabb does not mean that one falls
into doing something makrooh in principle, because what is mustahabb comes
under the heading of commands and what is makrooh comes under the heading of
prohibitions. The word mustahabb refers to something that is enjoined in
Islam but is not absolutely binding, such as Duha prayer. The word makrooh
refers to something that is forbidden by Islam, but not in a manner that is
absolutely binding, such as the prohibition on women following funeral
processions. Umm Atiyyah (may Allah be pleased with her) said: We were
forbidden to follow funeral processions, but not strictly so. 

Narrated by al-Bukhaari, 1219; Muslim, 938 

The one who does something that is mustahabb will be
rewarded, and the one who does something that is makrooh is not sinning. 

Based on that, it cannot be said that the one who does not
pray Duha has done something makrooh, just as it cannot be said that the one
who does not raise his hands when moving from one posture to another in the
prayer has done something makrooh. 

Ibn Daqeeq al-Eid (may Allah have mercy on him) said: 

We should note the difference between saying that it is
mustahabb to do such and such and saying that it is makrooh not to do it.
The one does not imply the other. It may be mustahabb to do a thing, but not
doing it is not makrooh, such as Duha prayer, for example, and many naafil
acts of worship. 

End quote from Ihkaam al-Ahkaam Sharh ‘Umdat al-Ahkaam,
p. 19 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

Not doing something mustahabb is not necessarily makrooh. 

End quote from Fath al-Baari, 11/17 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have
mercy on him) said:

Not doing something that is Sunnah does not necessarily mean
that one has fallen into makrooh. Hence if a person does not raise his hands
in prayer when bowing, he is not doing something that is makrooh. 

This is a basic principle: not doing something mustahabb does
not necessarily mean falling into makrooh. 

End quote from al-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘,
4/358 

And Allah knows best.