The Commons Clause is problematic because
- it makes the software non-Open-Source
- the resulting license is often ambiguous and possibly contradictory.
The first aspect is not a legal matter, but it's worth pointing out that the idea of Open Source as defined by the OSI is based on the concept of Software Freedom: the recipient's right to use, inspect, modify, and share the software for any purpose. To avoid confusion, it may be better to refer to Commons Clause licensed software as "Source Available".
The second aspect is more tricky, especially in the context of Copyleft Licenses like the LGPL. The core feature of copyleft licenses is that derivatives can only be published under the same license terms. For example, if you modify the GPLv2-covered Linux kernel, you can only share or sell your modified kernel under GPLv2-terms.
The good news is that the LGPL-2.1 copyleft provisions only apply to recipients, not to the original author. That is, if you created all of the LGPL-2.1 covered material yourself, then you are not bound by this copyleft provision, and could alter the license terms by adding the Commons Clause. However, the result would not be LGPL-covered in any meaningful way, and it could be confusing to recipients to describe the resulting license as LGPL.
This confusion might lead some recipients to invoke clause 10 of the LGPL-2.1, which says:
You may not impose any further restrictions on the recipients' exercise of the rights granted herein.
This could lead recipients to think that the Commons Clause terms are invalid and can be ignored. However, the "you" in that sentence is a recipient, and you as the original author would not be bound by this term.
When interpreting such unclear contracts, courts could apply a variety of techniques, depending on jurisdiction and context. They might try to figure out the intent, which here would be to disallow certain commercial uses. Or they might apply a doctrine like contra proferentem, that any ambiguity should be interpreted against the drafter of the contract. Here, the drafter would be you.
The effect of the Commons Clause is currently being litigated in the Neo4j v. PureThink case, which will be heard later in 2023. Neo4j published software under an AGPLv3 + Commons Clause license. This is slightly different to the LGPL-2.1 because the A/L/GPL-3 license family explicitly tells recipients that they can remove "additional restrictions" from the license, a term that is intended to prevent Commons Clause style restrictions. So the question is whether the resulting license is the AGPL (with additional restrictions that can be ignored), or a completely new license that overrides this AGPL terms.
In the face of this uncertainty, it would probably be wiser to avoid the Commons Clause, and instead get a lawyer to write you a custom Source Available license with the terms that you need.